Hyracotherium, also known as
Eohippus or the
dawn horse, is an
extinct genus of very small
perissodactyl ungulates that lived in the woodlands of the
northern hemisphere, with species ranging throughout
Asia,
Europe, and
North America during the early Tertiary Period and the early to mid
Eocene Epoch, about 55—45 million years ago
with the earliest fossil specimen found at the Tsagan Khushu Quarry 1 site,
Mongolia averaging about 60 cm in length and weighing around 15–16 kg.
This small, dog-sized animal is the oldest known horse and was once considered to be the earliest known member of
Equidae before the type species was reclassified as a
palaeothere, of a perissodactyl
family related to both
horses and
brontotheres.