The Boulder was originally going to be voiced by Dwayne Johnson and was based on his wrestling persona "The Rock."
Originally, Toph wasn't meant to be a girl. Her character started out as a heavily muscled guy, who was a bit of a mean-spirited jock, to better contrast against Sokka's supposed nerdiness. The prototype of her original design is depicted as the Earthbender in the opening title sequence and was later used for Roku's earthbending teacher Sud from season three.
The earth-bending tournament "Earth Rumble VI" resembles wrestling matches in most respects. "The Boulder", who is said to be the former "Earth Rumble" champion, is voiced by Mick Foley - an actual former wrestling champion, who had competed in a "Royal Rumble" tournament (in particular).
Toph (The Blind Bandit) shares many characteristics with Prince Adam (He-Man). Both were born in a rich and powerful family, both have amazing powers which they hide from their parents by pretending to be weak and incompetent, and to conceal their true identities they use only a different outfit and name. Interestingly, Toph's father is voiced by Cam Clarke who also provides the voice for Prince Adam/He-Man in He-Man and the Masters of the Universe (2002).
The Beifongs are the only characters in the show with last names.