Choose the word which is different from the rest.

The answer is D. Ass.

The other four animals (rhinoceros, lion, deer, and cow) are all mammals. Mammals are a class of vertebrate animals characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones.

An ass is a domesticated member of the horse family, Equidae. It is smaller than a horse, with a shorter head and neck, and a more sloping back. Donkeys are used as working animals in many parts of the world, and they are also kept as pets.

The other four animals are not asses.