A Roman relief depicting a bull on it's way to be sacrificed. Notice that this is only a part of the relief, their's also a pig and a sheep (thus becoming a traditional pig, sheep, bull sacrifice).
The origin of the Cretan bull is disputed among the ancient authors; some say that it was the bull which Europa rode, others that it was a gift to Minos from Poseidon. The later scenario is somewhat illogical as Minos was supposed to sacrifice the bull to Poseidon (it all seems very strange) but he was so charmed by its beauty that he kept it. Poseidon was angered by this and he put madness into the bulls mind and made it caused havoc to the land.
Erysteus asked Heracles to catch this bull and Minos allowed him to do so (I really wonder if Heracles would have taken no for an answer). The hero caught it and carried it home where he ended up setting it loose, allowing it to roam Greece until it stayed at the fields of Marathon (Theseus, an ancient king of Athens, later found it there).