There are 25 different types of creatures the player may encounter in the game, Fallout 3. These creatures can be humanoid, robotic or mutated versions of other animals.GameSpy: Planet Fallout Wiki

While there are a number of new original creatures in Fallout 3, many of the creatures made appearances in previous Fallout games. One of these creatures is the Super Mutant. These highly mutated humans have played an important role in almost every Fallout game and will continue to be one of the most difficult enemies the player must contend with in Fallout 3. Other creatures making a comeback include ghouls, giant ants, giant cockroaches and radscorpions, among others.No Mutants Allowed: Fallout 3 FAQ

Inspiration for the Super Mutant characters in the game, Fallout 3, actually came from a line from the movie Blazing Saddles according to the concept artist Adam Adamowicz, who sketched out their initial look. The line, "Don't shoot him, you'll just make him mad," summed up what the concept artist wanted to the Super Mutants to be - huge monstrosities that were more likely to get annoyed by bullets rather than be harmed by them. Adamowicz said, "I wanted them to look like they would step into a tree shredder, for relaxation. Their musculature was to be straining at the bone structure underneath, creating a hyper jacked up caricature of a person in the throes of radioactive testosterone poisoning, and liking it." Adamowicz also said this sort of attitude led the conceptual team to create some fun gags for the creatures, like the fact that the carried the remains of their victims in a shopping cart strapped to their backs.IGN: Bethesda Blog: Conceptual Design