Zeus, and Calyce. He is usually described as a king of Elis. He was loved by the moon, Selene, and this is usually explained as the fact to which his name alludes, since it derives from endyo, 'to go in', 'enter' (the same verb as Latin induco), this referring to the moon's seduction of the king.
It could also refer, of course, not to the moon but to the setting sun which 'enters' the sea, or else allude to the cave where Endymion met the moon. (In Hesiod's Theogony Night and Day live alternately in the same building, and some accounts tell of Endymion being a hunter who hunted at night and slept in a cave by day, so was thus always thought to be asleep.) So maybe it is the moon that 'enters' Endymion, or perhaps it is Endymion who finds himself 'entered', that is, encompassed or 'bathed' by his love the moon.
The reference to sleep could be significant, too, since Selene put Endymion to sleep for ever so that he should not die (or according to another tale, Zeus gave him a wish and he chose to sleep forever in a cave). It was this that some classical name explainers had in mind when they derived his name from the Latin somnum ei inductum, 'the sleep put upon him'.