Pteranodon, Stegosaurus, or T. Rex?
My personal suggestion would be to wait before you get any of them. Being at level 21, its possible that buying either the Stegosaurus or Trex it could drive your opponents ferocity beyond the point in which you can still win matches. It kinda depends on what you have now... I'd wait until you have a few other gold creatures & at least a couple other creatures who are equal in ferocity to what the trex or stego are at level 1, that way they are less likely to drive opponent difficulty up too much. As far as pteranodon, I dont see it as being worth its cost unless you have the disposable income. Trex and stego are both among the strongest tournament players, so they're both good buys. Trex is a very strong player and between it and stego, I'd get trex first. Hope that helps