Tyrannosaurus

They make a fuss out of the Allosaurus, but the Tyrannosaurus is completely ignored. Just after the scene where the Styracosaurus/Triceratops hybrid kills the Allosaurus, a Tyrannosaurus enters the scene and kills it. The Allosaurus jumps on the Styracosaur's back and tries to bite into it, but the Styracosaur throws it to the ground and gores it to death. The much larger Tyrannosaurus leaps onto the Styracosaur's back, but it is able to bite into the Styracosaurus. It eventually rips into the Styracosaurus' leg and finally kills it. It just goes to show you what would have happened to Kong 8 years later, had it been able to sink it's teeth into him.

And if you're going to say that it's just a larger Allosaurus, the skin is different, the arms are bent differently and it has only two fingers. I have no clue why the Kong T-Rex had three fingers though, given that it was scripted to be a Tyrannosaurus. I guess it's because O'Brien had Marcel Delgado do the model instead of making it himself. I still don't get why it was never noted as being in the film. It just kind of comes out of the blue.