I made up some 'Dire Rabbits' as mounts for a group of 'gnomads'. Sized them up, gave them a handful of HD (four, I think) and a secondary attack (kick, of course) and they thought I was nuts. The gnomads had mini lances and riding saddles to boot. Hilarious. The look on my player's faces when they got charged by a group of gnomes on the back of giant rabbits. Worth it, I tell you. Might have been able to put the warbeast template on them, come to think of it. Overdone..?