Sagittarius, Chiron

Chiron was eldest and wisest of the Kentauroi (Centaurs), a Thessalian tribe of half-horse men. Unlike his brethren Kheiron was an immortal son of the Titan Kronos (Cronus) and a half-brother of Zeus. When Kronos' tryst with the nymphe Philyra was interrupted by Rhea, he transformed himself into a horse to escape notice and the result was this two-formed son.
Zeus then placed him amongst the stars as the constellation Sagittarius or Centaurus.