Tough. As you subtly point out, not all dwarfism is island-related. But I mean, there are so many creatures that don't fit into the standard vertebrate model; single celled organisms, lichen and so forth.

If we stick to familiar 'animals', hmm... I think they're all up for dwarfism! I even checked 'ants' and found that to be commonplace, too =/ Good question lol