The Marathonian Bull, originally known as the Cretan Bull, was created by Poseidon for King Minos to sacrifice. But when Minos did not, Poseidon made the king's wife fall in love with the bull, and they had a son, the Minotaur.
Given to King Minos
When the king of Crete, Minos, wanted a sign from the gods, he pleaded Poseidon an animal for him to sacrifice. So the god of the sea sent a white bull, Minos liked bulls, using a wave and it landed on the dock. The people were amazed at this sight but Minos, so amazed by the bull, decided to keep it for breeding. He decided to sacrifice another bull despite the protests of Daedalus. Unfortunately, Poseidon noticed that the bull was different. As revenge, he went to Aphrodite to have Minos' wife, Pasiphaë, fall in love with it. It worked but the bull didn't feel the same way towards Pasiphaë. So she had Daedalus make a fake-cow suit and successfully made love with it. Nine months later, Pasiphaë gave birth to the Minotaur, even though Minos named it Asterion after his stepfather, the former king.
Hercules' Seventh Labor
As the seventh labor for Hercules, Eurytheus told him to bring the Cretan Bull to him. Much to Hercules's surprise, Minos said he can have it almost immediately. After the birth of the Minotaur, the bull was now a constant reminder of the king's shame and disgrace. When Hercules brought the bull to Eurytheus, the latter told him to take it to the city and released it. Reluctantly, Hercules did as was told and the Cretan Bull rampaged around the Greek countryside near an area called Marathon. It was renamed from the Cretan Bull to the Marathonian Bull.
Death from Theseus
When Theseus finally arrived at Athens, he found it being involved with many bad things, including the Marathonian Bull. To get an audience with King Aegeus, his stepfather, without revealing his identity, Theseus decided to kill the bull. During a storm, he went to a hut owned by a woman named Hecale, a former priestess of Zeus. She reveals that the storm is due to the god of lightning himself, since the bull was sacred to his son, Minos, Zeus won't let anyone kill it. So Theseus promises to sacrifice to Zeus here when he's done, this results in the storm going away. Since the bull needed to be brought back alive, Theseus lured it into a barn where it got caught in the snares. But when Theseus return to the hut, he found that Hecale had passed away overnight. So Theseus built a domed monument in her honor and sacrificed the Marathonian Bull himself at the temple of Zeus at Athens. Much later, he defeated the bull's son, the Minotaur, in Crete.
The bull was massive with white fur and gleaming white horns.