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.
Chris Bradford/Dogpound/Rahzar (Teenage Mutant Ninja Turtles 2012 TV series)
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.
Daxter (Jak and Daxter series) was first transformed into an ottsel by exposure to Dark Eco, and further exposure granted him the power to transform into Dark Daxter.
Warren Worthington III/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.
Remy LeBeau/Gambit's (Marvel Comics) secondary mutation manifested as a hypnotic charm.
After the original spider bite that gave him his powers, Peter Parker/Spider-Man (Spider-Man: The Animated Series) eventually began to mutate further, becoming the monstrous Man-Spider.
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...
After first becoming a vampire, Michael Morbius (Spider-Man: The Animated Series) was transformed into a giant bat after being exposed to the Neogenic Recombinator.
The Ninja Turtles (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.
Donatello (Teenage Mutant Ninja Turtles 2003 TV series) underwent a secondary mutation after being stung by one of the S-Gray Mutants.
In the Mutant Apocalypse timeline, Leonardo (Teenage Mutant Ninja Turtles 2012 TV series) became the super-mutant Maximus Kong after being caught in the explosion of a mutagen bomb.
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.
Chaos (Sonic the Hedgehog) was once a normal Chao, but was mutated into his current form by prolonged exposure to Chaos Energy.