The old Latin word for horse is equus . Besides the horse , there are seven other species of genus Equus in the Equidae family . These are the ass or donkey , Equus asinus ; the mountain zebra , Equus zebra ; plains zebra , Equus burchelli ; GrÃ vy 's zebra , Equus grevyi ; the kiang , Equus kiang ; and the onager , Equus hemionus .