Some Others I would like to add:

In the game data, there is a character for Eggman out of his mech, with his own moveset. He even has his own losing animation!

While you can hack to play as Rocket Metal and Eggman in his UFO thing, they do not have unique movesets and instead default to Knuckles' moveset.

If you hack to play as them, Metal Sonic and Eggman (without the mech) actually have their names rendered in the end ranking screen, which possibly indicates that they were intended to be played, but were scrapped at some point.