Since 2006, new field studies at the late Oligocene site of Quebrada Fiera (Mendoza, Argentina) have increased the previous faunal record from these levels that are presently considered as the base of the Agua de la Piedra Formation. Former faunal lists included Pyrotherium and Proborhyaena gigantea, representatives of the Deseadan Age. Among new materials, a complete skull and two associated metatarsals, two maxillary fragments with left I1-M2 and right I1-P3, and five isolated upper teeth represent the Family Notohippidae. Even though some characters are not comparable between the skull and maxillaries, shared features lead us to consider them as a unique and new taxon, Mendozahippus fierensis, gen. et sp. nov., and the isolated molars as cf. M. fierensis. As in previous phylogenies, our analysis indicates the Family Notohippidae is a paraphyletic group. The inclusion of Plexotemnus, Pampahippus, and Puelia within Notohippidae is not supported, but without them, the group is still paraphyletic. Leontiniids appear as basal taxa to the remaining notohippids and toxodontids. Mendozahippus presents one autapomorphy (lingually projected protocone of M3) and four homoplastic characters. It occupies a basal position with respect to other notohippids plus toxodontids. Mendozahippus increases the diversity and geographic distribution of Notohippidae during the late Oligocene. This and other taxa add some distinctiveness to this Deseadan fauna compared to other better-known assemblages from Patagonia or Salla (Bolivia), whereas other groups maintain clear affinities. The intermediate latitudinal position of Mendoza might explain these faunal characteristics.