What fire-breathing bull was captured by Heracles as one of his labors?

The fire-breathing bull captured by Heracles as a labor was the Cretan Bull.

Poseidon sent the striking white bull to King Minos of Crete, who was supposed to sacrifice it. Minos kept the animal instead, angering Poseidon. The god’s punishment helped set off the famous story in which Minos’s wife Pasiphaë fell in love with the bull and later gave birth to the Minotaur.

The bull then became savage and ravaged the Cretan countryside. Heracles was ordered to capture it alive as his seventh labor. He subdued the animal, brought it back across the sea, and delivered it to Eurystheus. Eurystheus released it, after which it wandered to Marathon and became known as the Marathonian Bull, eventually killed by Theseus.

The fire-breathing detail is prominent in later retellings and is not equally emphasized in every early source. That variation does not change the canonical answer: the Cretan Bull. It should not be confused with the fire-breathing Khalkotauroi, the bronze bulls faced by Jason in Colchis.