Theseus was the mythical king of Athens and was the son of Aethra by two fathers: Aegeus and Poseidon. This monster is probably inspired by the sea monster that is present in the storyline of his Six Labours: in a location named Megara, there was an elderly robber that forced travellers along a narrow cliff-face pathway to wash his feet. He then kicked those travellers off the cliff so that they were eaten by a sea monster. In this tale, Theseus was the one that pushed that robber off the cliff.
This card's name may also come from the thought experiment known as the Ship of Theseus, which questions whether an object (specifically a ship) that has all of its parts replaced remains the same object.