What three-headed dog did Heracles capture as his final labor?

Heracles captured Cerberus as the final labor of his twelve labors.

Cerberus was the monstrous watchdog of Hades, guardian of the Underworld’s gates. Greek sources describe him as multi-headed, most famously with three heads, though artistic and literary traditions sometimes give him more. He was said to be the offspring of the giant Typhon and Echidna, making him kin to other famous monsters such as the Hydra and the Nemean Lion.

To complete the task, Heracles descended into the Underworld after being initiated into the Eleusinian Mysteries. Hades permitted him to take Cerberus only if he subdued the creature without weapons. Heracles used his strength and lion skin to overpower the dog, then brought it before Eurystheus, who was terrified and hid in a storage jar. Heracles eventually returned Cerberus to Hades.

Cerberus is often confused with Orthrus, another monstrous dog in Greek mythology. Orthrus had two heads and guarded the cattle of Geryon; Cerberus guarded the dead.