1850, Amer.Eng., "untamed or half-tamed horse," from Sp. bronco "rough, rude," originally a noun meaning "a knot in wood," perhaps from V.L. *bruncus "a knot, projection," apparently from a cross of L. broccus "projecting" + truncus "trunk of a tree."
Online Etymology Dictionary, © 2010 Douglas Harper
This critter is from the "rough stock" herd, her job is to send a bareback cowboy sailing. She really looks quite friendly-like waiting for the rodeo to begin.