In Guardians of the Galaxy Rocket asks Peter Quill to retrieve a prisoner's leg and he also attempts to get Ravager's cybernetic eye later in the movie. What's with his obsession for artificial body parts?
They're both jokes. Raccoon asks for the leg just to mess with Peter (and that prisoner) and he asks for the eye later as a call-back to that joke.
Raccoon: Oh, I was just kidding about the leg. I just need these two things.
Raccoon: I thought it'd be funny! Was it funny? What'd he look like, hopping around?
Raccoon: There's one more thing we need to complete the plan: that guy's eye!
Peter: No, we don't need that guy's eye!
Raccoon: No, seriously, I need it! [giggles]
That's just his sense of humor. He saw a guy with a prosthetic eye and couldn't resist. It's a part of his (and the franchise's) irreverent and sarcastic nature.