1The American red squirrel, Tamiasciurus hudsonicus.
2In extended use: a person who lives in a mountainous area; a hillbilly. [Applied especially to inhabitants of the Appalachian region of the eastern U.S. Occasionally derogatory, though sometimes also a self-designation.].
3The collared lizard, Crotaphytus collaris.
Mid 19th century. Probably from mountain + boom + -er, apparently arising from the folk belief that the animals so named are capable of making a booming noise.