Betsie is a T-Rex that reproduces whenever she takes damage.
Freddie and Betsie have a relationship of some sort, which it might be why she constantly gives birth. They both may be in a pack with Betsie being the alpha female. Sally once confused Betsie for Freddie, and cried when Billy killed her.
When this man was a young boy at camp, Betsie came to the park and killed nearly everyone, leaving the man as the only survivor. He then spent years trying to find the dinosaur to make sure she could never harm another living soul again. Strangely enough, he was not actually very determined to kill the dinosaur, gladly quitting after finding an awesome car.