Fantasy Magical Creatures

The difference between a creature and a person in a costume is whether the non-human parts look attached or grown.

That comes down to matching. Horns that share a tone with the hair. A tail whose fur matches something else on the body. Wings that sit where shoulder blades would be rather than floating behind. None of that is difficult, and almost nobody does it on the first run, which is why most creature characters look like people wearing a headband.

Get the matching right and the clothing barely matters. Get it wrong and no wardrobe will save it.