I'm wondering, Shorty appears to be an orphan and have an unknown past. Do you think that he has some form of PTSD? I can guess that, seeing as how Littlefoot was acting after his mother's death, that Shorty must have gone through something similar, if not worse, in his life.
His meanness might be a coping mechanism. What do you think happened to him? Did he lose his parents to Sharpteeth? Did he lose them to an accident? Was he abused by them and hence ran away? Are his parents still alive but he hasn't seen them in years and doesn't know where they are or if they are still around? Is he, in fact, Littlefoot's long lost older brother? Or do you think it's something else?