Cerberus, the three-headed dog or serpent:

This star pattern has always been associated with the constellation of Hercules. According to mythology Cerberus was the three-headed dog who guarded the path to the realm of the Underworld. As one of his Twelve Labors, Hercules had to capture Cerberus and bring him back to King Eurystheus. The constellation of Cerberus has its origins with the astronomer Johannes Hevelius, who placed it on his star atlas Firmamentum Sobiescianum. Most representations of the constellation have it appearing as a three-headed serpent instead of a dog. Cerberus is no longer recognized as a separate constellation, instead its stars are now part of Hercules.