Leton is an archaic term for brass used in alchemical texts. A collection of alchemical treatises by John Houpreght titled Aurifontina chymica refers to it, noting that Hermes referred to it as Terra foliis.
Design
Leton's depiction as a massive white bird of prey likely is in reference to a set of instructions contained with Aurifontina chymica that states the material must be "whitened" through the process of Albedo. Albedo was occasionally symbolized with a white swan or eagle in alchemical art.