A Poncho! This is some sort of cheap plastic 1-size-fits-all raincoat. Usually these are give-aways if it tries to rain on you at an amusement park. Where in the USA could this have come from?
This keychain isn't officially Sonic, but it was a part of a Sonic promotion when the Sega Dreamcast came out. You can sort of see the reddish/orange-ish swirl on it.