Rexes are one of the regular inhabitants of Dinosaur Land, and probably didn't willingly assist Bowser. It is more likely that they was just trying to get a bite to eat, and foolishly thought that Mario looked tasty.
"As one would expect, you could only meet this kind of enemy in dinosaur land, and there are lots of them too. You have to stomp on a Rex twice to defeat him. Rex has wings, but I don't hink he can fly." - SMW Instructions.
"It's easy to vex Rex. Stomp once to crunch him like an accordian and twice to get rid of him for good. A Spin Jump takes him out in one step." -Mario Mania
Attack Pattern: He usually moves straight forward until he falls or hits something, although sometimes he's smart enough to turn around. After being stomped once, he starts to charge.
Basic Strategies: You can beat Rex in one easy step by stomping him. A cape attack, fireball or Yoshi slurp will work too.
Advanced Strategies: You'll get a coin for beating him with a fireball or eating him (no points, but coins are better anyway). You can earn the most points by stomping him, and then trying to land on him again without touching the ground (600 pts total).
