Who were Cerberus's parents?

Depending on sources the siblings of Cerberus include Orthos, the Hydra of Lerna, Khimaira, the Sphinx, the Hesperian Drakon, the Caucasian Eagle, the Crommyonian Sow, Gorgon, the Dragon of Colchis, Scylla, as well as the otherwise unnamed Echidnades (son of Ekhidna).