Hyracodon nebraskensis (Leidy 1856; Eocene–Oligocene, 32-26 mya; 1.5 m length) was originally considered the founding member of the Hyracodontidae. The dentition resembled that of later rhinos, especially Paraceratherium, the largest of all land mammals, which now nests with horses.

Uintaceras radinskyi (Holbrook and Lucas 1997; Late Eocene, 45 mya) nests between Hyracotherium and Hyracodon at the base of modern rhinos and their kin. Formerly it was considered a species if Fostercooperia, which nests at the base of the indricotheres and Pappaceras. Note the drawing by Holbrook and Lucas (here colorized) does not exactly match the specimen (photo) also colorized. Paleontologists need to be more careful.