What giant serpent guarded the oracle at Delphi?

The giant serpent that guarded the oracle at Delphi was Python.

In Greek mythology, Python was a huge serpent who presided over the oracle at Delphi, a sacred site believed to mark the center of the world. The Greeks associated that center with the omphalos, or “navel” stone, which Python guarded. The creature was connected with Gaia, the Earth, and represented an older chthonic power at the sanctuary.

Apollo later became Python’s great enemy. According to the Homeric Hymn to Apollo, the god killed the serpent with his arrows and claimed the Delphic shrine. Other accounts say Python pursued Apollo’s mother, Leto, before Apollo was born. After the slaying, Apollo was said to have undergone purification, and the sanctuary’s priestess became known as the Pythia.

Python is not the same creature as the Hydra, which Heracles fought at Lerna, or Ladon, the dragon that guarded the golden apples. The name also survives in modern biology: the snake genus Python was named after the mythological serpent.