The Radioactive-Dinosaur Spider is a giant mutant Tyrannosaurus with six spider legs (one eye in each of the middle legs), huge venomous fangs, and a head with eight eyes. It was most likely created by the method seen in Jurassic Park, or the doctor used a time machine to capture a live T-rex.
- It first appeared in the episode "Honorary Freak", where Entrée tried to have it bite Patrica, turning her into a mutant. It just chased her throughout the jungle.
- In "Sgt. Snuggums", it appeared in Joe's flashback locked up in a cage in the doctor's lab.
- It is one of the doctor's two prehistoric mutants, the other being Allopotamus.