Shellos is a slug-like Pokémon whose appearance changes drastically depending on the sea it is found in. However, both forms have a shell-like back and an amorphous lower body. Both forms also have yellow lining around their eyes, mouth, and back.
In the West Sea form, the underside is white and its shell is pink. The shell has several small spike-like protrusions on it. On its head is a ring of pink fleshy knobs resembling flower petals that spike up.
In the East Sea form, the underside is green and its shell is blue. The shell has two flap-like protrusions. These flaps are lined in yellow. On its head are two white-tipped knobs resembling horns 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 form Shellos' appearance is theorized to have been influenced by warm ocean waters. While the East Seas Shellos is believed to have been influenced by cold ocean waters. It is also believed its diet influences Shellos' two forms but 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 it is pushed or squeezed, a mysterious purple fluid will ooze out of its body which is said to be sticky and similar to greasy sweat. 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 sea shore. Shellos is said to have great regenerative capabilities. Its diet consists of eating plankton. Shellos can in both land and sea, though it stays on land until its skin dries.
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.