Megareus, like Megara, has a name that means 'shrine'. He was a son of Poseidon by Oenope (or else of Onchestus, Aegeus, Hippomenes or even Apollo). He was king of Onchestus in Boeotia, and went to Megara (significantly) to aid Nisus in his war against Minos, king of Crete. His name could be taken to refer to one of the shrines at Megara, where perhaps he may have prayed for victory. (He was in fact killed in battle.)