hyracotherium
Concise Oxford English Dictionary © 2008 Oxford University Press:
hyracotherium /ˌhʌɪrəkə(ʊ)ˈθɪərɪəm/
▶noun the earliest fossil ancestor of the horse, a small forest animal of the Eocene epoch with four toes on the front feet and three on the back.
'hyracotherium' also found in these Oxford entries:

