The Teumessian Fox is a monster of Greek mythology, a gigantic fox sent to prey on the children of Thebes. In the Marvel fictional universe it is mentioned as being one of the offspring of Echidna but it has not yet made an appearance in Marvel Comics.
Background
The Teumessian Fox is a descendant of Set, an Old One. It's the offspring of Set's granddaughter Echidna who mated with Typhon, one of the Titans. The giant fox which was destined never to be caught, was sent by the gods to punish the inhabitants of Thebes by preying on its children. The regent of Thebes trained Laelaps, magical dog which is destined to catch anything it pursues to hunt the fox. Resulting in the two ending up in an eternal chase.