In the interest of bumping the thread and getting to know characters more, I'm answering my own question. That, and long-winded answers are fun.

I feel that Clay's book counterpart is Tobias, although Clay's less of a bullying victim and more of a bully. That, and Clay has a bit more home-life angst and a bit less trapped-as-a-bird angst (A lot less trapped-as-a-bird-angst). Tobias becomes a hawk, which allows him to quite literally fly away from all of his problems. On the other hand, I feel like Clay's less flighty (Hah puns) than to choose to run away from home with his morph.

Between his tendency to snap after hitting a boiling point and his status as a bully/delinquent, I feel like Clay's biggest redeeming quality is that he's fiercely protective/loyal, and likes to guard people and things that are important to him. Because of this, I like to think that Clay would lean towards canine morphs. Although there's temptation to give him something cool like a Dingo or African Wild Dog, Clay sticks out for his bad reputation throughout the small town, so I think he'd want a morph that doesn't stick out too much/is kind of cuddly and likable.

Those factors, as well as his size compared to his peers, and other plot stuff to be revealed, Clay gets the [url=http://www.coolanimalworld.com/wp-content/uploads/2012/11/How-to-feed-Caucasian-Shepherd-Dog-3.jpg]Caucasian Shepherd[/url].

EDIT: If it's just Theo and Clay who get cool explanations for signature morphs, I will cry for a thousand years.