Hyracotherium leporinum was a small, very early odd-toed ungulate. It was once presented as the archetypal ‘dawn horse’, but current phylogenetic work places it close to the base of Perissodactyla, before the major split between the horse lineage and the tapir–rhinoceros lineage. It is therefore more useful to picture it as a primitive perissodactyl in its own right than as a miniature modern horse.

Its low-crowned cheek teeth combine rounded cusps with developing transverse crests. That dentition is suited to cropping and processing relatively soft vegetation, so browsing on leaves, shoots and fruit is a reasonable dietary interpretation rather than evidence for grazing. A small, terrestrial and fairly active animal is plausible by comparison with other early perissodactyls, but the London Clay species itself is known much better from its skull and teeth than from its limbs.

The type is a fragmentary but uncrushed skull with a substantial upper tooth row, making cranial and dental anatomy unusually informative for such an early mammal. The cheek teeth are also notably variable, which is why the species is best understood from combinations of characters across the tooth row rather than from a single worn molar.