When Ixion seduced the Hera illusion, the illusion gave birth to Centaurus (a depraved human boy). Centaurus then mated with the mares of Mt. Pelion in Magnesia, giving birth to the race of centaurs.
Note: Though Chiron is often referred to as King of the Centaurs, he is not a descendant of Centaurus, but rather the son of Cronus and Philyra.