Poor Rex. This neurotic green dinosaur may look tough, but he's actually one of the most timid and insecure toys in Andy's toy box. One of the first things we see him doing is panicking during the birthday party scene:
REX: But what if Andy gets another dinosaur? A mean one? I just don't think I can take that kind of rejection.
We feel you, Rex. Change is scary.
Rex knows that he should be ferocious, but feels kind of insecure about not being able to roar and intimidate other toys. He also hates confrontation. When the other toys start to turn on Woody, Rex doesn't really want to take sides. Eventually, he gets peer pressured into rejecting Woody, which doesn't make him feel good.
Later, he has tons of guilt once he realizes that Woody wasn't actually trying to kill Buzz. It's okay, Rex. Give yourself permission to make mistakes, big guy.
Wallace Shawn provides the voice of Rex for all the Toy Story movies. You might recognize him from tons of other movies he's been in like My Dinner with Andre or Clueless. And would we forget The Princess Bride? Inconvincible!