Tweety was not specifically named in the original cartoon, but staff called him Orson.
Mickey Mouse was originally to be named Mortimer by Walt
Disney until his wife suggested the friendlier-sounding “Mickey.”
Pluto was originally Minnie’s dog Rover. He then became
Pluto a few months after the discovery of Pluto in 1930.
Goofy was originally Dippy Dawg for a few years until 1934,
when he became Goofy.
Elmer Fudd was originally Egghead until the 1940 cartoon
“Elmer’s Candid Camera.”
Mighty Mouse was Super Mouse until another cartoon
character by the same name came out. It was changed to Mighty to
Betty Boop was originally called "Nancy Lee" or "Nan
McGrew" in a few episodes. She was also originally a poodle. In
her first official color cartoon, she was actually portrayed as a
redhead. Later she was changed back to black hair and a red dress.