There's a fair bit of history on Rocket Raccoon on the various wikis, but none seem to answer one fairly specific question - why is he a raccoon? Wikipedia gives an out-of-universe explanation, saying "The character was created by Bill Mantlo and Keith Giffen and consciously inspired by the classic Beatles song "Rocky Raccoon"".

Is there an in-universe explanation as to why an extraterrestial civilization had a raccoon as their guardian?