The man who "invented" punching slabs of beef for training is in the movie… and it isn't Rocky, even though he says he invented it. Heavyweight boxing champion Joe Frazier, who cameos in the movie, grew up in Philly, and was known for pounding slabs of beef. (Source)

There are two Stallone family cameos in Rocky—one is a mangy mutt Stallone does a favor for… and the other is Stallone's dog Butkus. Sylvester Stallone's musical brother, Frank, is the acapella singer in the alley, and the dog Stallone adopts is played by his real-life dog. (Source)

Rocky's real name is "Robert," allegedly according to a draft of Rocky II. Rocky is simply a nickname. And it looks a lot better on a movie poster than Robert. (No offense to any Bobs out there.) (Source)