The Dromomerycids were a family of deer/antelope-like ruminates that lived during the Miocene... I think the majority of them were native to North America.
The three-horned monster is <I>Procranioceras skinneri</I>, while the four-horned beast behind it is <I>Sinclairomeryx riparius</I>. The small, ring-horned buck is <I>Rakomeryx sinclairi</I>.