Theseus and Pirithous and the Calydonian Boar

Theseus and Pirithous were among the company of heroes that hunted the Calydonian Boar.  The Calydonian Boar is one of the monsters of Greek mythology that had to be overcome by heroes of the Olympian age.

The Calydonian hunt. Tondo of a Laconian black-figure cup, ca. 555 BC.

King Oeneus ("wine man") of Calydon, an ancient city of west-central Greece north of the Gulf of Patras, held annual harvest sacrifices to the gods on the sacred hill. One year the king forgot to include Great "Artemis of the Golden Throne" in his offerings Insulted, Artemis, the "Lady of the Bow", loosed the biggest, most ferocious boar imaginable on the countryside of Calydon. It rampaged throughout the countryside, destroying vineyards and crops, forcing people to take refuge inside the city walls (Ovid), where they began to starve.

Oeneus sent messengers out to look for the best hunters in Greece, offering them the boar's pelt and tusks as a prize

Calydonian Boar Hunt , Frieze from the Ashmolean Museum, Oxford