From Creatures Wikia
The Rex is a medium-sized carnivorous Dinosaur that is native to KT. It is a highly dangerous predator, and will actually attack, kill, and eat Norns when given the chance, and will also lay eggs. It is also unmovable by the Hand, making it difficult to get rid of.
KT's expansion, The Bone Cave , features a tool that can convert the Rexes into harmless vegetarians, therefore removing their threat to Norns.