User who has already been mutated in some way develops a "secondary mutation," often gaining new powers or expansions on their existing ones. In some cases, the user's body mutates even further as well.
The secondary mutations can either be naturally or artificially induced.
After undergoing brutal experimentation in Iron Heights, Killer Croc (Batman: Arkham/DC Comics) mutated even further, developing such things as a tail.
Mohinder (Heroes) mutates after injecting himself with a faulty formula to gain an ability.
Angel (Marvel Comics) gained Healing Blood from his secondary mutation, a phenomenon experienced by several other mutants like him, in which they gain new powers and/or appearances.
Emma Frost's (Marvel Comics) diamond form is the product of her secondary mutation.
Gambit's (Marvel Comics) secondary mutation manifested as a hypnotic charm.
Under mysterious circumstances, the Standard Zombies (Half-Life Series) can mutate into full-fledged Gonomes.
Depending on the virus strain in their bodies, zombies (Resident Evil) can mutate further into either Crimson Heads...
The TMNT (Teenage Mutant Ninja Turtles; 1987 TV series) have developed further mutations that cause them to transform into monstrous hulks with superhuman strength, lowered intelligence, and increased aggression.
After initially becoming the mutant Dogpound, Chris Bradford (Teenage Mutant Ninja Turtles; 2012 TV series) became the more agile and deadly Rahzar after further exposure to mutagen.