T. rex Tail
The tail of T. rex and other dinosaurs did a lot more than just counterbalance the front end. Dinosaur tails housed a massive set of limb muscles that powered their every step. My work on digitally reconstructing those muscles suggests that dinosaur tail power was much greater than we thought.
Persons WS IV, Currie PJ. 2011. The tail of Tyrannosaurus: reassessing the size and locomotive importance of the M. caudofemoralis in non-avian theropods. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology 294: 119–131.