BrontotheriumBron`to*the"ri*um\, n. [NL., fr. Gr. ? thunder + ? beast.] (Paleon.) A genus of large extinct mammals from the miocene strata of western North America. They were allied to the rhinoceros, but the skull bears a pair of powerful horn cores in front of the orbits, and the fore feet were four-toed. See Illustration in Appendix.