古兽马（学名Palaeotherium），又名古兽，是一属已灭绝的原始奇蹄目动物。乔治·居维叶（George Cuvier）原先认为牠们是一类貘，故牠们一般都被描绘成一细小的貘。近年就其头颅骨的研究发现牠们的鼻腔并非用来支撑鼻子，故现开始将牠们描绘成更像马的动物。解剖学研究亦发现古兽马，连同其他古兽马科的属，包括始祖马，与马是近亲。

古兽马的物种平均约有75厘米肩高。牠们生活在4千5百万年前的始新世早期至中期，当时被热带雨林覆盖的欧洲。最大的物种P. magnum是生存于始新世中期的法国，差不多等同一只马的大小。

Palaeotherium ('old beast') is an extinct genus of primitive perissodactyl ungulate. George Cuvier originally described them as being a kind of tapir, and as such, Palaeotherium is popularly reconstructed as a tapir-like animal. Recent reexaminations of the skulls show that the nasal cavity was not designed to support a small trunk, thus starting a recent trend to reconstruct them as looking more horse-like. Recent anatomical studies also suggest that Palaeotherium, along with other palaeothere genera such as Hyracotherium, were closely related to horses.