>>1077205
It's never explicitly stated in-universe, but there are implications that Professor Gerald was very familiar with echidna lore from ruins at the Mystic Ruins and on Angel Island. For example, he was able to create a replica of the Angel Island Emerald Shrine on the Space Colony ARK. He was also able to create Artificial Chaos, presumably based on the murals of Chaos in the Mystic Ruins. There was a mural of Super Sonic fighting Eggman on Angel Island, in Hidden Palace Zone in Sonic and Knuckles. Shadow's spines naturally stick up like Super Sonic's. Eggman looks almost exactly like Gerald, so he also could have seen it as a mural of himself creating Shadow, or Shadow giving him an Emerald. I know this sounds like pure headcanon, but note that Sonic Adventure 2 contains actual plot points that you can only see if you pause a cutscene (which you can't do in game) and read an in-game newspaper and computer screens. Anyway, the whole thing with Gerald going to Angel Island and creating things based on it was made explicitly canon in Shadow Generations, which came with a physical copy of Gerald Robotnik's Journal, a book that Eggman talks about a bunch in Sonic Adventure 2 and a few later games. It's "redacted" and missing pages, with the in-universe explanation that GUN recovered it from an abandoned Eggman base, but it explicitly has Gerald talking about going to Angel Island and making Artificial Chaos and stuff based on what he found there.
Alternatively, you could say it's pure coincidence. But most fans, including the current writer, seem to think it was intended by the people who made Sonic Adventure 2, and he acts as if it's canon and implies it further. But it's kind of cool that it's not explicitly stated.
>>1077207
Check out issues 1-12 of the IDW Sonic comics. Metal Sonic is cool as hell in that. If you have the patience for ArchieSonic autism, you can check out the saga of Shard. The original Metal Sonic reformed and became a good guy, fighting his replacement (who is for all intents and purposes just regular Metal Sonic, exactly like Shard was originally).
>Sonic the Hedgehog #25
Sonic CD adaptation, so the first appearance of Metal Sonic.
>Knuckles' Chaotix (One-Shot)
Very loose adaptation of Chaotix, but does have Metal Sonic and his cool transformation.
>Sonic the Hedgehog #86-87
Essentially the return of Metal Sonic. Important to his character development.
>Sonic the Hedgehog #236-240
The original Metal Sonic, the one with an actual personality, returns in this arc and becomes a good guy, calling himself Shard. And it leads into the next arc where he is more important...
>Sonic Universe #41-44
"Secret Freedom." Shard and a team of other characters I don't care about do stuff.
>Sonic the Hedgehog #241
Epilogue to the Secret Freedom arc.
>Sonic the Hedgehog #245-246
Admittedly Shard isn't terribly important here, but he doesn't have that many appearances and the stuff he does here does contribute to his character arc.
>Sonic Universe #50
A special issue all about Metal Sonic. Really it's about (the current and final) Metal Sonic fighting Shard. It's also the final issue before the universe gets semi-rebooted and becomes a lot less retarded.
But I just realized you said you thought the design was cool. When Metal becomes Shard, he is rebuilt and ends up looking like Metal Sonic 3.0, from Sonic Rivals 2 (although they are unrelated story-wise, and in fact the Metal Sonic Shard ends up fighting is called 3.0, though again, unrelated to the one from Rivals 2).
Other Metal Sonics, including the one who fights Shard, do appear, but usually they're just random mooks. However, you might be interested in...
>Sonic Universe #55-58
"Pirate Plunder Panic." Immediately following the semi-reboot universe rewriting stuff, Amy and Cream end up in Blaze's universe, where the previous Metal Sonic was left for dead, and presumably forgotten, because in the new history, there was only ever one Metal Sonic. He only wasn't erased because Blaze's Jeweled Scepter (from Sonic Rush Adventure) is what creates alternate dimensions, and thus semi-protected her dimension from the re-write. Anyway, he gets rebuilt with scrap parts and now looks like a pirate, so he goes around calling himself Captain Metal, and he fights Blaze, Amy, and Cream. Again, his design is a little different, because he's dressed like a pirate now, but I think it's pretty cool.
Now if you really want cool Metal Sonic stuff, check out the UK's Sonic the Comic. Totally different continuity, also makes some big changes from the game, but ends up being much less retarded than Archie, and has some cool stories. The most popular stories revolve around Metal Sonic. For some reason he's called Metallix in this comic, but whatever.
>"The Sonic Terminator" in Sonic the Comic #24-28
This comic had multiple stories running every issue, but each story was shorter. So this four issue story is more like one or two issues of an American comic. Anyway, this is the Sonic CD adaptation. Aside from calling Metal Sonic Metallix, it's pretty standard.
>"Total Chaotix" and "The Brotherhood of Metallix" in StC 53-62
These are called two stories, but one leads directly into the other. They're widely seen as the best arc in the comic. They make some changes, like the big red Metal Sonic transformation from Chaotix is now a different robot built by an army of Metal Sonics to be their leader. They get up to time travel stuff that is really cool.
The Metallixes appear many more times, but usually just as regular enemies. Interestingly, the comic introduced a Knuckles Metallix, essentially a Metal Knuckles, before Metal Knuckles was introduced in Sonic R. But the Knuckles Metallix are still usually just treated as regular minions.
Archie published a Sonic X comic, ostensibly set in the universe of the series, but actually the timeline doesn't line up exactly right. But it was actually pretty good. Anyway, in Sonic X #24, Eggman makes essentially this universe's version of Metal Sonic, but it's colored white and not given an official name. But it's kinda cool.
Of course, you can also watch the OVA.