The Mutant Jellyfish fought Zilla Junior in the episode "Future Shock". In the episode it is seen tracked down by H.E.A.T. and Zilla Junior. When H.E.A.T. came back from the future Zilla Junior already destroyed it with his jaws.
Regular Abilities: Very weak acid globs from the mouth
Special Abilities: Fast swimming
Oddly enough, the Mutant Jellyfish looks nothing like a Jellyfish.