Given the description of what is required to make someone a fiend I don't see why one might not have the head of a cute animal instead.
They're personality is mostly tied to the 7 sins so all that is required is a link between one of these and the animal you want.
So the person have been a cowardly/suspicious individual in life and now in death they have a rabbits head and are always standing tall when they think they are being observed and refer to run away and hide when confronted and strike hidden from the shadows
I would generally assume that only carnivorous animals (including omnivores like bears and boars, and carrion-eaters like vultures and hyenas) would be choices, but I do love the idea of rakshasa who have non-threatening herbivorous animal heads, like one with a doe's head or a rabbit's head or a dove's head, to catch people off-guard, perhaps being prone to pretending to 'be one of the good ones' and tricking humans into all sorts of perfidy, out of sheer wickedness.
(The original picture that spurred the idea, IIRC, from Dieties and Demigods, showed rakshasa with a tigers head, a boars head and some sort of bird head.)
We haven't seen any rakshasa with insect, fish or reptile heads. That would be neat, one with a crocodile head, or a praying mantis head.
I love the idea that the original animal-headed 'gods' of some quasi-Egyptian setting were rakshasa, taking advantage of their relative invulnerability to mundane weapons and low-level magic, immortality and innate sorcery, to lord it over the humans of that kingdom...