To unlock "Tea-Rex", you must complete the fifth chapter in Deleted Scenes, Dino Cup.
To unlock the Dino Cup, you must have beaten the first four cups in the deleted scenes. Not all stars are required, just enough to unlock the cup.
There are five levels, featuring a variety of stars but only three "Pro" medals.
You do not need to obtain any of the optional stars to complete the level, you must just finish each level in turn. The achievement will unlock as you cross the finish line in the final scene, "Attack of the Cones".
Any questions, please ask.