Going to post this in bits because I'm still working through it all (also there are image limits). Someone will have to handle the technical side because things get complicated. Sonic Retro's chao page is incomplete. It's combining every game into one and not talking much about the history, so here's some history, a.k.a. "all the things you couldn't get because you're not Japanese". In rough chronological order, starting with... Sonic Adventure (Dreamcast) Chao are kind-of a guessing game on the Dreamcast because you never see the stats on-screen. If you up the speed, a chao starts looking vaguely like Sonic, if you up the flight, it starts looking vaguely like NiGHTS, but emphasis on "vague" and as it's part of normal gameplay, isn't very special. There's also the gold/silver/black eggs you can find in the adventure fields, but again, well known, not very interesting. But Sega teased a special type of Chao, "Light Chaos" (ライトカオス) Sega gave no clues on how to get this, and apparently it took until May 1999 for someone to work it out. I'm not sure it was even mentioned in the West, probably because the mystery had been solved months before you got to play the game, but yes, this is the rarest Chao in Sonic Adventure. Except for all the other ones. Fun fact: they put this Chao in Sonic Runners, and I'm guessing 99% of the audience had no idea what it was.
The very first downloadable Chao came at Tokyo Game Show '99 Spring on 20th March 1999 (the first day for the public). Sega had some sort of booth, where if you brought your VMU, you could get a copy of what they were calling the first "Jewel Chao" (ジュエルチャオ). https://retrocdn.net/index.php?title=File:DCM_JP_19990416_1999-13.pdf&page=104 This is "Emechao" (エメチャオ). It's an emerald-coloured Chao... and that's about it. This is the first of what Sonic Team would call "Rare Chao" (レアチャオ), and the only way to get it was to go to TGS. They seem to have some special Dreamcast software specifically for transferring this Chao - I don't think that's ever been seen online, let alone dumped, so I guess in a way you could call it a long lost Sonic "game".
You could see their stats on Chao Adventure. Interesting. I was under the impression that Emechao was available on the online Chao Black Market based on this, but I guess that page is wrong? Do we know what was available on the SA1 Black Market?
This is Rubychao (Rubichao?) (ルビチャオ). It was supposedly distributed at Dai 10-kai Jisedai World Hobby Fair on 12th July 1999, except we don't have any scans to prove this. There were VMU things at that WHF but whether you could get this specific Chao is a fact currently lost to time. And that was it for Sonic Adventure. The game supports other colours, and you can breed chao to get new shades, but as far as officially released Chao, you just get emerald and ruby. Unless there's others that haven't been documented. Sega ran a dedicated Chao BBS at the time, so chances are there are plenty more to talk about, but for those not invested in the internets, that's as far as it goes. Until Sonic Adventure 2.
According to us (and others), after these physical events had occurred, Sonic Team made these Chao available to download through the Sonic Adventure's online features. On the US launch, Ruby Chao became available (1999-09-09), and on the EU launch, Eme Chao (1999-10-14). 23rd December 1999 also introduced "Safa Chao" (Sappha Chao?) (サファチャオ). None of this was well reported at the time. There was indeed the Chao black market that opened at some point, but I don't know what it sold. And remember, this is SA1, not SA2.
Sonic Adventure 2 (Dreamcast) SA2 came a little too late to jump on the "rare chao" bandwagon - within two months of release Sonic Team were already plugging the GameCube conversion, so I don't believe there were any physical events where you'd get special Chao. There was however, as mentioned, a black market page for SA2's online service. Here's a backup of it, and yes, the gang's all here, including new additions, "Amechao" and "Moon Chao", all of which require a prerequsite number of emblems. Not sure if these could be brought over to the original Sonic Adventure - in theory they might be compatible, but you'd have to check (though obviously the gold/silver/black ones were available in that game from the get go). In SA2B, the "vague looking Sonic" chao can be made to look slightly more like Sonic, and if you use a dark chao, it starts looking like Shadow. I genuinely don't know if that's a feature of the Dreamcast version. Someone should check.
Sonic Adventure 2 Battle (GameCube) Knuckles Chao was downloadable at GameJam2 in Tokyo International Forum: Sega Spring Collection on 30th March 2002. Now we're on the GameCube, visitors were expected to bring Game Boy Advances along - anything that had the Tiny Chao Garden (or "Chao no Petit Garden" since it's Japan)... which in March 2002 was just Sonic Advance. The program looks remarkably similar to the Dreamcast version though. The Knuckles Chao is different from the Chao released for the Dreamcast, in that it already had stats. Sonic Team were clearly thinking more about this concept in this iteration of the game - it's not just a change of colour, it's a whole bespoke design that was unobtainable outside of this event (although not for long). For what it's worth, anything that works in SA2B also works in Sonic Adventure DX. This was not the case with their Dreamcast counterparts.
Future mascot of Sonic CulT, Amy Chao was downloadable at Dai 16-kai Jisedai World Hobby Fair on 13th July 2002. Same deal with Knuckles, but with Amy. I should probably point out, these WHF events occurred in more than one location in Japan, which means Amy was more widely distributed than Knuckles. And again, we pre-date any other Tiny Chao Gardens than Sonic Advance.
Here's the part where things get messy. Every weekend between 20th July and 8th September 2002, Sonic Team held a Waku Waku Game Party event, where you could download rare Chao. I think they flip-flopped between Knuckles and Amy, i.e. nothing "new" in this initial run, but it means these are probably the two least-rare rare Chao. A second set of Waku Waku Game Party events occurred between the 2nd November and 22nd December 2002 with a slightly different game lineup, but again with the same rare Chao. Although there was an addition: "Tsuyatsuya Sonic Chao" (つやつやソニックチャオ). It's a shinier version of the "vaguely looks like Sonic" Chao. I can't tell if this replaced Amy and Knuckles, or was distributed alongside them, but you couldn't get it before the 2nd November.
What about Tails? This Tails Chao wasn't distributed at an event. Instead you get it through playing Phantasy Star Online Episode I & II, which launched in Japan on 12th September 2002. You can also get it in the Plus version of the game, and I'm guessing(?) the Western versions? That means it might be the only rare Chao available overseas, except given PSO is an online game, maybe it's a faff in 2023. You tell me.
At Dai 17-kai Jisedai World Hobby Fair on 19th January 2003, Sega again brought the Knuckles, Amy and Tsuyatsuya Sonic Chao along, but this time introduced "Tsuyatsuya Shadow Chao" (つやつやシャドウチャオ): Shiny Shadow. This again seemed to vary depending on the venue, so I can't guarantee its first outing was on the 19th, but it couldn't have been before then. Sonic Advance 2 was out by this point, so that's another option for transportation. Later on, at Dai 18-kai Jisedai World Hobby Fair, starting on on 21st June 2003, Sonic Team brought Knuckles, Amy and Tsuyatsuya Sonic and Shadow, and introduced a new Chao: "Tsuyatsuya Tails Chao" (つやつやテイルスチャオ) I think this was the last Chao to be released at an event, so no "Tsuyatsuya Knuckles" or Rouge or Eggman variants.
How do you get these Chao today? Hack the game to bits. Or if you want a more legitimate method, there's "Nintendo GameCube Software e-Catalog 2003: Enjoy Plus-ban" (ニンテンドーゲームキューブソフトeカタログ2003 エンジョイプラス版), a GameCube demo disc we don't have a date for. Now you can get the Tails, Knuckles and Amy Chao in the comfort of your own home. Provided that home has a Japanese GameCube. Conclusion: The following are the only true "Rare Chao": - Sonic Adventure: "Eme Chao", "Ruby Chao", "Sappha Chao" - Sonic Adventure 2 Battle (and SADX): "Knuckles Chao", "Amy Chao", "Tails Chao", "Tsuya Tsuya Sonic Chao", "Tsuya Tsuya Shadow Chao", "Tsuya Tsuya Tails Chao". No I'm not sure about the romanisation. Everything else lies, except for Light Chaos, which is special for a different reason (until it stopped being special). Though I can't rule out other special events occuring between 1998 and 2003 - there were a lot. Also it's been a very slow work day.
Chao compatibility between SA1 and SA2 is partial and (probably) not an intentional feature. Back in the day I tried transferring the Gold Chao from SA1 to SA2 and it basically worked, but it lost its coloring. Never tried SA2 > SA1, though.
Just to prove there were (some) differences in software - at many of these events, the area to get Chao was called the "Rare Chao Download Corner" (レアチャオ ダウンロードコーナー): The 15th WHF The 17th WHF. The 18th WHF. Some other event. For the record, I've yet to come across an official source calling these things "Character Chao", which is what the English speaking world seems to have settled on. Spoilers: you're probably wrong.
There never could have been Rouge or Eggman Chao, since all the special Chao models were coded into the game at release. They clearly intended for Knuckles, Amy, and Tails to be the only ones. For what it's worth I extracted the GBA multiboot ROMs with the Chao preloaded from the demo disc many years ago, and I think they might work in any region if you have the means to download a file from your PC to a GBA (possibly via a modded Wii?), or using Dolphin's GBA compatibility. For PC players, you can use krzys2's Chao Editor to manually turn a Chao into a special one, or the famed Chao World Extended mod implements new methods for unlocking these Chao in SA2. There's also a version of it for SADX that allows for special Chao to be obtained.
perhaps, though the name isn't particularly useful if it's also used to refer to sappha chao and et all, which aren't based on characters like the sa2 ones are
Slight amendment - I didn't live in GameCube land and the wiki glosses over this: A game like Sonic Advance, which includes a Tiny Chao Garden, was not a prerequisite to obtaining Rare Chao - you can still put them on your Game Boy Advance. This is because as pointed out, Sonic Adventure 2 taps into the GBA's "multi-boot" technology, basically storing a Chao garden in the GBA's RAM. Whenever I've heard of this feature, it was in the context of multiplayer games, i.e. being able to compete against your friends without owning multiple cartridges. Apparently it was viable for data transfer too. The problem with this method is you need to keep the GBA running. Without a cartridge, you can't save, so as soon as you lose power, the RAM will be cleared and you'll lose your Rare Chao. Turns out they thought of this: There's a "sleep mode" which will extend battery life, and this is what Sega were advocating you use, should you not have Sonic Advance with you. Nothing seems to suggest these events used different software for the GBA - I'm not sure why you'd want a full Chao garden if the plan was to get home before the battery died, but whatever. Then again this is the original model GBA that apparently could score 15 hours of regular play on two AA batteries, so maybe not a problem.
In the Plus versions, some of the previously online-only quests were available offline. The GBA games were available offline. I don't remember if all of them were, but a random search seems to indicate that the chao garden was present. There was NiGHTS Score Attack, a version of Chu Chu Rocket that contained all of the puzzles from the Dreamcast version, the Chao Garden with Tails, and a Puyo Puyo game. I believe Billy Hatcher also contained some GBA download link games as well? I did dump all of the PSO games back in the day, including the Chao garden. It took about 5 to 7 minutes per game (I still have the original data with timestamps) because the private dumping software that one of the GBA homebrew devs provided me with would write it to SRAM on a GBA cart which I'd then have to backup to a PC. But each multi boot game is 256KB and the GBA cart only had 64KB of SRAM so I had to download each game 4 times, extract the data from each, then combine them together at the end. I believe all of those dumps are still readily available today, especially since when Plus came out they could just be extracted from the disc. But at the time, dumping them the hard way was the only option.
There is also another form of convenience when it comes to downloading the Tiny Chao Garden to a GBA without Sonic Advance. SA2B/SADX will create a copy of the Chao you are transferring and send it to the GBA while keeping the original Chao safe on the GameCube, just in case the GBA loses power. In that scenario, only the copy will be lost while the original Chao is kept safe. When you send the Chao copy back to the game it came from, the original will be overwritten with the copy. Given that Chao can travel between SA2B and SADX using a GBA, you could take advantage of this feature to clone your Chao, though I've never actually tried doing that. Could be something for me to try as I have all the necessary equipment to do so, or someone else could test it.
I'm not entirely sure if it was available in the regular North American version of PSO EP1&2 GC, but I do remember getting this Chao with the non-Plus EU release and Action Replay. But unfortunately, to a degree that's counting on my memories of like 15 years ago, and it's highly likely that this was accessed online via a private server and I entirely forgot. EDIT: Okay, looking at the chao island guide this is possible in the original release of pso ep1&2 in offline mode: Tails Chao Guide - Chao Island (chao-island.com) I think I might still have my memory card with my tails chao on it lol
