Not much is known about the T-Rex. He only appears in Jumpy Ghostface's illusion he seems to be male and red in color and (like some animals) has the abillity to speak human.
In Resurrection of T-Rex, Twin Masters found a caveman and a T-Rex frozen in a block of ice and uses his magic on them in a plot to destroy Big Green. Due to First Squad being more advanced, they defeated the caveman and the T-Rex. When Twin Masters attacks, T-Rex "sacrifices" it's life to "eliminate" Twin Masters.
- Judging by its physiology, the fossil which inspires Jumpy to imagine a T-rex does not appear to be a T-rex itself.