始祖马（学名Hyracotherium），又名始行马或始马，曾被认为是马科中最早的成员。现时牠被认为是属于古兽马科，这是与马及雷兽有关的科。始祖马的大小如狗一般，生存于北半球，分布在始新世早期至中期（距今约6千万至4千5百万年前）的亚洲、欧洲及北美洲。

始祖马的第一个化石标本是由古生物学家理查·欧文于1841年在英格兰发现，他基于牙齿的缘故而怀疑这是一头蹄兔目。他并没有发现完整的骨骼，故将牠命名为「似蹄兔兽」（Hyracotherium），即始祖马的学名。于1876年，奥塞内尔·查利斯·马什在美国发现了一个完整的骨骼，并命名为曙马（Eohippus）。后来发现这两个发现非常接近时，始祖马因最先被发表，故曙马则成为了异名。

Hyracotherium (/ˌhaɪərəkoʊˈθɪəriəm, -kə-/ HY-rək-o-THEER-ee-əm; "hyrax-like beast") is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates which was found in the London Clay formation. This small, dog-sized animal was once considered to be the earliest known member of Equidae before the type species, H. leporinum, was reclassified as a palaeothere, a perissodactyl family basal to both horses and brontotheres. The remaining species are now thought to belong to different genera, such as Eohippus, which had previously been synonymised with Hyracotherium.