I was hoping at least someone would've told me what he was. 
I couldn't figure it out.
He's a Nothronychus, according to a post from the official LBT FB page. (So glad they made him look more like a proper therizinosaur than the yellowbellies!)
I gotta admit, I actually like Wild Arms. I can see where his hyperactive/cowardly antics would be annoying to some people, but for the most part, I thought he was quite funny! The scenes where he plays off Mr. Threehorn are gold!
And when he and Chomper meet...THAT got actual laughter out of me! (Few LBT movies have ever made that happen.) I think part of the humor of him is that he's not the type of character you'd typically find in the LBT universe. They have him use more modern language like "up top," but they don't overdo that kind of thing to the point where it goes from funny to annoying. Actually, Wild Arms seems like what you would get if you dropped some normal, modern-day dude into the dinosaur world--while other characters are like, "Oh, it's a sharptooth, it must be Tuesday," he's flying into a panic. Let's face it: if any of us got dropped in the LBT world, we would react in the same way as Wild Arms.
Can't help but suspect Wild Arms will continue to feel he owes Bron for his kindness to the point that he will feel independently moved to save someone Bron cares about. He'd be the type who'd be dragged kicking and moaning into adventures while, in spite of himself, coming to like these adventuring people.
Ever since XIV, I've had a headcanon quite similar to this. I really don't think we can say Wild Arms is completely selfish. He may not have wanted to lead Grandpa Longneck and Mr. Threehorn to the Fire Mountain, and he may have tried to convince them to go back early on, but as soon as Grandpa Longneck mentions the children to him, Wild Arms is defeated--he knows he can't argue with that, not wanting the kids to be in danger. Maybe he also felt guilty over it being his fault Littlefoot knew how to get to the Fire Mountain in the first place. So even as important as saving his own skin is to Wild Arms, I can't help thinking his good side ultimately overcomes that.
I do wish, however, that he'd gotten his proper chance to be an unexpected hero before the movie's end, helping to fend of a sharptooth or save Bron. Felt like he should have gotten that sort of character arc. Also, it would have been nice to see more of him and Etta interacting, seeing as how they seemed to be good friends from the brief moment of screen time they had together, plus Etta's reaction to Littlefoot mentioning him.
(Oh, and I've noticed, by the way, that there must be some person on the internet who downright hates this character. The wiki descriptions of Wild Arms I've seen paint him in a very negative light, all but calling him an antagonist!)
