The Brontoth is a Maxis-made creature, present in Spore Galactic Adventures. They are large, T. rex-like creatures with many spikes on their backs and mouths. Brontoths exhibit sexual dimorphism; which the male's skin is darker, the females are lighter in color, are the same size, and have fewer spikes.
Brontoth appeared in the expansion pack's trailers, battling an epic Al Packa. All three kinds of Brontoth are listed as "gaprop", but none of them appear in any Maxis adventures. They were probably added because the trailer made them popular.