From mythopedia.com:
Centaurs were originally all male. Female Centaurs, or “Centaurides,” were probably introduced no earlier than the fifth century BCE, and even then, they were regarded as novelties.[12] Female Centaurs did sometimes appear in later art and literature: Ovid, for instance, tells of a female Centaur named Hylonome, whom he may or may not have invented.[13]