De uparrettåede hovdyr er en orden af pattedyr. I modsætning til de parrettåede hovdyr er de oftest kendetegnet ved et ulige antal tæer. Ordenen omfatter tre familier med nulevende arter, heste (Equidae), næsehorn (Rhinocerotidae) og tapirer (Tapiridae) med i alt omkring 17 arter. At disse meget forskelligt udseende dyr er beslægtede, blev som den første erkendt af den britiske zoolog Richard Owen i det 19. århundrede.
2 July, 2005, New insight into horse evolution Citat: "...North America was thought to be home to as many as 50 species and subspecies of horse..." It looks like, as far as we can tell from the DNA, there is only evidence of two species in North America," Dr Alan Cooper..."Probably these animals are adapting to local environments and perhaps they are [anatomically] more [changeable] than the palaeontologists had perhaps thought."..."