I'm going out on a limb here, but it might come from his mother. The name "Sycorax" has been interpreted as a combination of "sys" (sow) and "corax" (raven), making her a "sow-raven". If her form was, in fact, half human, half cut-and-paste animal, then it would make sense for Caliban to be part beast as well.
Well, it's never implied in the play, and the half-and-half theory was just something I thought of when I considered your interpretation. It'd be cool to see a stage production attempting to utilize it.
Did you happen to see the New York Met opera's new production "The Enchanted Island"? It was a blend between "The Tempest" and "A Midsummer Night's Dream" as a Baroque pastiche opera. Their rendition of Caliban was in this vein... I love your designs! They're fantastic!
A google image search of "met opera enchanted island" yields some pretty good results. Of course, Sycorax's costumes got a ton of attention because she had three of them over the course of the opera; her character transforms from an old hag to a beautiful but powerful sorceress. Anyway, you should take a look-- they're really interesting deigns.
Yes, I think I like this one even more. Somehow I don't think warm colours suit Caliban, and I like this palette much more. The skull paint on his face makes the skulls around his neck seem more in-place.