In Greek mythology, the Cretan Bull was the splendid bull Poseidon sent to Minos as a sign of kingship. When Minos refused to sacrifice it, Poseidon maddened the beast and made Pasiphaë desire it; their union produced the Minotaur. Heracles captured the bull as his seventh labour; later it ranged Attica as the Marathonian Bull until Theseus sacrificed it at Athens.
Adapted from Wikipedia, "Cretan Bull" · CC BY-SA 4.0
I.Poseidon's sign and the Minotaur
To prove his claim over his brothers, Minos asked Poseidon for a bull from the sea and vowed to sacrifice it. Poseidon sent a snow-white bull; Minos kept the animal for his herds and offered an inferior substitute. In anger the god afflicted Pasiphaë with lust for the bull and drove the beast wild across Crete.
With Daedalus' help Pasiphaë coupled with the bull and bore the Minotaur, which Minos later confined in the Labyrinth. The bull itself continued to ravage crops and walls until Heracles' arrival.
II.Heracles and Theseus
As his seventh labour Eurystheus ordered Heracles to fetch the Cretan Bull. Minos gladly allowed its removal. Heracles mastered the animal and shipped it to Tiryns; it later broke free, wandered to Marathon, and was called the Marathonian Bull.
Attic saga ties the same bull to Androgeus' death and Athens' tribute to the Minotaur. Theseus captured it on the road to Marathon—sheltering with Hecale on the way—and sacrificed it to Athena or Apollo at Athens before sailing to Crete to kill the Minotaur.