A T. rex, or Tyrannosaurus rex, is a large, extinct dinosaur. When Phineas, Ferb and Candace were trapped in the Late Cretaceous period a T. rex destroyed the time machine that brought them and chased them. They eventually lost it, but it found them again. When they were back in the present at the museum the T. rex accidentally came with them. It tried to attack Candace but Doofenshmirtz's Freezeanator hit it and froze it. ("It's About Time!")