Mayhem is a dog-like creature with golden-yellow fur as a base color, and blue-green fur on the top of its head, back and tip of its bushy tail (with the latter two being printed in a flame-like design). It has a jagged red fur-line on the top of its head, as well as the same red color around its red-brown eyes, resembling eyelashes.
Mayhem also has a red nose, a red fur patch around its neck, pointed ears (much like a cat), sharp claws, and sharp teeth with two fangs curving out, similar to a traditional Eastern dragon.
Mayhem has the ability to teleport itself and other objects (like Donnie's tech-bo) at will.
- April O'Neil: April rescued Mayhem from Baron Draxum, and now lives with her as a pet. The two of them apparently have a deep bond.
- Raphael: Mayhem scratched him during their first meeting, despite the fact that Raph was trying to be nice to him.
- Baron Draxum: Mayhem was somehow in possession of a vial belonging to Draxum, who appeared to hold a grudge against Mayhem as a result.
- Foot Clan:
- Mayhem is named after show writer Russ Carney's pet cat.