Gastrodon is a Pokémon based on a sea slug, and appears much the same as its pre-evolution, but it gains six visible but stubby legs and is noticably larger. They both additionally have plain black eyes with white irses, with a third eye in the centre of their forehead. Like Shellos, Gastrodon has two forms;

In the West Sea form, the flower-like crest it had as a Shellos has now turned into horns adorning the side of its head, which point directly upwards in a crescent moon shape. Its underside up to the bottom of its mouth is pink, its back and the top of its head is completely brown, and it has two with small protrusions on its back with several pink spots dotted around them. It also has two small protrusions on its neck underneath the horns, and it has a yellow border running down its body from its head, separating the brown and the pink coloration.

In the East Sea form, it retains the horns it had as a Shellos, although now fully coloured green which point at a 45 degree angle. Its underside up to the bottom of its mouth is blue, its back and the top of its head is completely green, and it has two yellow "fins" adorned on its back with a third bordering its side and rear, as well as two small blue spots between them. It has a yellow border running down its body from its head, separating the blue and the green coloration.

Gastrodon and its pre-evolution Shellos are examples of allopatric speciation, though they are still able to breed.