The Magic School Bus S02 E03 "The Busasaurus" (Dinosaurs)
In this episode, the kids visit Ms. Frizzle's old friend, Dr. Carmina Skeledon (Rita Moreno), at a dinosaur dig. Dr. Skeledon lets Arnold see a fossilized dinosaur egg as the kids are leaving, and he still has it when they go back in time. An Ornithomimus steals the egg, and both Arnold and Phoebe chase after it. The rest of the class is left to find them, and in the end, Arnold squares off with a T-Rex after being grown.
This is the animated version of portions of The Magic School Bus In the Time of the Dinosaurs.