Shellos is a sea slug-like Pokémon whose appearance changes drastically depending on the sea it is found in. However, both forms have amorphous, slug-like bodies. Like real sea slugs, they have mantles covering the top halves of their bodies. Unlike real sea slugs, they have bulbous heads and four small feet rather than one large foot. Both forms also have black oval-shaped eyes, tiny noses, wide yellow lips, and yellow outlines around their eyes and mantles.
In the West Sea form, the underside is white and the mantle is pink. The mantle has several small spike-like protrusions on it. On its head is a ring of pink fleshy knobs resembling the gills of a nudibranch.
In the East Sea form, the underside is green and the mantle is blue. The shell has two flap-like protrusions, resembling cerata. These protrusions are lined with yellow. On its head are two white-tipped tentacles resembling rhinophores that extend away from its face.
The Nurse Joy featured in Gateway to Ruin! revealed Mt. Coronet's formation led to the allopatric speciation event, with the original population of Shellos being separated into two. As a result, Shellos' color and other physical differences began to vary due to environmental differences and population isolation over time. West Sea Shellos' appearance is theorized to have been influenced by warm ocean waters, while East Sea Shellos is believed to have been influenced by cold ocean waters. It is also believed that diet influences a Shellos' form, but this has yet to be proven. The two forms are still able to interbreed; the child takes the mother's form or the father's if bred with a Ditto. If Shellos is pushed or squeezed, a mysterious purple fluid, said to be sticky and similar to greasy sweat, will ooze out of its body. In the anime, West Sea Shellos can extend their necks to incredible lengths. It is unknown if the East Sea form can do this as well. Shellos live along the seashore. They are said to have great regenerative capabilities. The species’ diet consists of plankton. Shellos can live on land or in the sea, though its skin dries out if it stays on land for too long.
Shellos's most notable appearance is that of a companion of Mr. Fuego's. He typically keeps his East Sea Shellos on his shoulder, continuing to do so even after he accidentally imprisoned himself to avoid Team Galactic. It first appeared in Maddening Magby.