Gastrodon is a sea slug-like Pokémon. Much like its pre-evolved form, it has a slug-like body with a round head and a mantle covering the top half of its body. It has six stubby legs and three black eyes with white irises. A yellow stripe runs along the edge of its mantle, through its lower two eyes, and around the back of its head. Like Shellos, Gastrodon has two forms based on the sea it lives in.
In its West Sea form, it has a pair of large tentacles on its head that curve upward, as well as a smaller, straighter pair of tentacles on its neck. Its underside is pink, while its mantle is brown with three pink spots on each side. There are two lumps on its back, resembling the cerata on real-life sea slugs.
In its East Sea form, it retains the head tentacles it had as a Shellos, which are now fully green and angled slightly more upward. Its underside is blue, while its mantle is green with two blue spots on each side. It has a pair of yellow, fin-like cerata on its back and a wavy yellow line bordering its side and rear.
Gastrodon and its pre-evolution Shellos are examples of allopatric speciation, though they are still able to breed. It is also a relative of Shellder and Cloyster. When it is threatened, it oozes purple fluid and flees. Gastrodon lives in shallow tidal pools. Its body is soft and squishy, yet surprisingly resilient. It has strong regenerative capabilities that allow it to recover from injures within a few hours. Its diet consists mainly of plankton, but it is occasionally known to look for food on land, leaving behind a trail of sticky slime. However, Gastrodon cannot stay on land for too long due to its body being highly vulnerable to dehydration. In ancient times, it had a shell on its back, which gradually became a thin yet hard vestigial plate.
West Sea - It has strong regenerative capabilities. Even if parts of it are bitten off by fish Pokémon, it will return to normal within a few hours. East Sea - A team of researchers is scouring the oceans of the world in search of a Gastrodon in a color never seen before.
West Sea - Plankton, invisible to the naked eye, is its main food source. It comes onto the land periodically, but the reason for this is not known. East Sea - Experiments are being conducted to discover what happens when a Gastrodon is raised in a location other than the sea where it was born.
Gastrodon and its pre-evolution were accidentally released in Pokémon GO on April 29, 2019, but were removed shortly after. On May 17, 2019, they made their official debut along with various other Pokémon from Fourth Pokémon generation.
Gastrodon was featured alongside several bovine Pokémon, namely Tauros, Miltank, and Bouffalant, in multiple promotional artworks created to celebrate New Year's 2021, also known as the Year of the Ox.