As I think about it, that's a pretty good rule for most vertebrates(I exclude fish in this). If the newborn can walk around within an hour of being born or hatched, it's a lot more likely to be incredibly cute. Deer, giraffe, elephant, horses are all pretty cute. Kangaroos(all marsupials, actually), rats, mice are all pretty ugly. You could start a flame war debating the relative cuteness of newborn humans, canines and felines in comparison. Though they get pretty cute fairly fast.