OPHIOTAUROS was a monster born with the foreparts of a black bull and the tail of a serpent. It was slain by a giant ally of the Titanes who hoped to sacrifice its guts in a rite which would have ensured victory against Zeus. However the god's kite stole the parts away, thwarting the plan. Ophiotauros was presumably set amongst the stars as the constellation Taurus (which was sometimes envisaged as a serpent-tailed bull), alongside the kite as Lyra, and the altar Ara.
This story may have been part of the lost Greek epic known as the Titanomachia (War of the Titanes) in which the giant Aigaion appeared as an ally of the Titanes. Ovid names Aigaion Briareus in this passage who was actually his son in both the Titanomachia and the Iliad epics.
GAIA (Ovid Fasti 3.793)
CLASSICAL LITERATURE QUOTES
Ovid, Fasti 3. 793 ff (trans.Boyle) (Roman poetry C1st B.C. to C1st A.D.) :
"The Kite star Milvus . . . If you want to know what bestowed heaven on that bird : Saturnus [Kronos (Cronus)] was thrust from his realm by Jove [Zeus]. In anger he stirs the mighty Titanes to arms and seeks the assistance owed by fate. There was a shocking monster born of Mother Terra (Earth) [Gaia], a bull, whose back half was a serpent. Roaring Styx [an ally of Zeus] imprisoned it, warned by the three Parcae (the Fates) [Moirai], in a black grove with a triple wall. Whoever fed the bull's guts to consuming flames was destined to defeat the eternal gods. Briareus [Aigaion] slays it with an adamantine axe and prepares to feed the flames its innards. Jupiter [Zeus] commands the birds to grab them; the kite brought them to him and reached the stars on merit."
- Ovid, Fasti - Latin Poetry C1st B.C. - C1st A.D.