Mabon ap Modron--"Great Son son of Great Mother"
Welsh:
mabon: from mab: youth, son, young man, man.
ap: son (decendant of) (also mab, map, fab, ap, vab, ab)
modron: mother (believed to be cognate with the Gaulic "Matrona"--mother, matron)
Mabon ap Modron is the Welsh god of music, love, and fertility. He represents the change of seasons, and can be equated with the Greek goddess Persephone. He is depicted as a prisoner, such as in the story of "Culhwch ac Olwen" in the Mabinogion, where he is set free by Cei and Bedwyr, the prototypes of Kay and Bedevere, to capture the boar Twrch Trwyth.
The story says that Mabon ap Modron is the best huntsman in the world, but stolen from his mother when he was three nights old; it isn't known if he is living or dead. Arthur and his knights must find Mabon, so that he can hunt Twrch Trwyth, an evil man turned into a murderous boar, and then present the boar to Yspaddaden Penkawr, father of Olwen, who refuses to allow Culhwch to marry her until the boar is caught.
His prison is Caer Llowy--Gloucester, but also thought to mean "City of Light." He can only be found by consulting the Salmon of Llyn Llyw, the oldest animal in the world. (Salmon often represent wisdom; cf. Fionn Mac Cumhail.) Cei and Bedwyr ride the salmon to the prison; on being set free, Cei carries Mabon on his back to King Arthur's court. From there, they hunt the boar, Mabon gaining the comb and shears behind its ear, and then driving it off a cliff into the sea around Cornwall.
As for an interpretation of this... The boar is an otherworldly animal, perhaps a symbol of the darkness that Mabon, as the light, must hunt and drive out. Moreover, it is a boar which gored to death Adonis--perhaps an end of the tale not told is that Mabon is killed in slaying the boar. It is Mabon who facilitates the fertility of the land, symbolized by the marriage of Culhwch and Olwen.
Mabon is often equated with the Irish god Oengus mac ind Og, "Angus the Young Son," ruler of Tir na nOg, the Land of Youth. He is born on the winter solstice (a supposition based on the Newgrange monument), son of the Dagda and Boann. He is the god of love and music.
His main traits can be summerized as the following:
Stolen from his mother
Imprisoned (Held at Caer Llowy)
Is found on (or around) the winter solstice
Is a champion hunter
Conjectured:
Is killed (or imprisoned) by his or his mother's enemies?
Other characters share his traits: Pryderi, Cuchulain, Fionn mac Cumhail, Peredur/Perceval, Gawain (decending from the Cuchulain figure), Lancelot, Owein ap Urien (his mother is sometimes listed as Modron), Merlin, Lleu Llaw Gyffes, and Galahad.