LEON was a leonine Gigante (Giant) slain by Herakles during the war of the giants against the gods. The hero made a cloak of his skin.
A lion-headed Gigante does occur in the ancient Pergamon relief depicting the War of the Giants. However, this myth is really a variant of the tale of the Nemean Lion which was the usual source of Herakles cape.
Presumably GAIA (see the Gigantes)
CLASSICAL LITERATURE QUOTES
Ptolemy Hephaestion, New History Book 5 (summary from Photius, Myriobiblon 190) (trans. Pearse) (Greek mythographer C1st to C2nd A.D.) :
"Herakles did not wear the skin of the Nemean lion, but that of a certain Leon (Lion), one of the Gigantes (Giants) killed by Herakles whom he had challenged to single combat."
- Ptolemy Hephaestion, New History - Greek Mythography C1st - 2nd A.D.