Timmy was not one character, but instead an interchangeable set of characters who "assisted" Mr. Lizard in his experiments on Dinosaurs. Each Timmy would come out, cheerfully start to help with the experiment, and be blown up, eviscerated, have his head blown off, or some other gruesome fate. Mr. Lizard, unfazed, would simply respond, "We're going to need another Timmy!" to hoorahs from Baby Sinclair and Earl Sinclair.
Timmy first appeared in the second season episode "The Golden Child."