I always thought her horse was fitting. Aside from the obvious farm reference, but when I think of a horse, I think of the open plains and John Wayne and Clint Eastwood and the merging of animal and human for a common goal. In the wild they are very symbolic of free spirits, and Cassie was, I think, the most different than the rest of the animorphs. That is to say, she was the farthest removed from their average qualities of do what needs to be done attitude and didn't go with the crowd like Tobias or Ax.