That's called a Atroxosaurus, they took it out from the final version. Though with a cheat that allows to control dino nests, if you select a t.rex nest, it will appear with the rider saddle, but no rider. Trust me, I would've loved that unit too.
It's from a boosterpack, like I said, although it does come with some maps. In Para-Welt, they have some maps created by some users, including myself, but alot of them come from the Non-english forum. You can also download the boosterpack from there. You can find the cheats there too.