I'd probably make a half ball, like this.
depends on how you want to do it, if you want him standing, and with his head raised, then you will need much more suppot going on. making him more sitting down, head on the ground would be easier.
standing, i would make legs, neck and head out of rice krispie treats. i think to support the head and kneck, you would be looking af screwing/ soldering things together - think ace of cakes style metal pipe supprt.
Whatever you decide to do can you please post a picture. I would love to see how it turns out, since littlefoot use to be my daughters favorite movie when she was little.