Most heavy animals walk on four legs.[1][2] There are some exceptions. Some really heavy birds move or moved on two legs.Moas are one example andostriches run very well on two legs.
Birds generally are an example of the change fromquadruped walk (originally asdinosaurs), tobipedalism. Humans are also examples of changes that started inarboreal apes.
The study of such things is called "comparative foot morphology".
↑Griffin T.M; Main R.P; Farley C.T. 2004. Biomechanics of quadrupedal walking: how do four-legged animals achieve inverted pendulum-like movements?.J Exp Biol.207 (20): 3545–3558. doi:10.1242/jeb.01177.PMID 15339951
↑Weissengruber G.E; & Forstenpointer G. 2004. Shock absorbers and more: design principles of the lower hind limbs in African elephants (Loxodonta Africana).J. Morphology260: 339.
