Shellos are slug-like Pokémon. Both forms have yellow lining around their eyes, mouth and back.

In the West Sea form, the underside is white, its back is pink in colour and has small spike-like protrusions on it. On its head is a ring of pink fleshy knobs arranged like a flower.

In the East Sea form, the underside is green, its back is blue and has two flap-like protrusions. These flaps are lined in yellow. On its head are two white-tipped knobs resembling horns.

Shellos and its evolution Gastrodon are examples of allopatric speciation, though the two forms are still able to interbreed (the child takes the mother's form).

It is shown in the anime that Shellos can extend their necks to incredible lengths. This has only been seen in the West Sea form. It is unknown if the East Sea form can do this as well, but it is most likely able to.