Gulpin is a round, green, amorphous Pokémon. It has a black diamond marking on its back and a yellow feather-like adornment on its head. A female has a shorter feather than a male. Its face consists of puckered lips and slanted eyes that are usually closed. It has two stubby appendages that are the same shape as its lips. It swallows anything of its own size whole since its stomach comprises most of its body. Its heart and brain are very small in comparison to its stomach. The unique stomach acid it has allows it to digest almost anything. When it is digesting, vile, overpowering gases are expelled. It lives in grasslands and savannas.
In Volcanion and the Mechanical Marvel, a Gulpin was living at the Nebel Plateau. It was particularly close to the Volcanion guarding Nebel Plateau. Sometime ago, it had been abandoned by a Trainer whose last act before departure was to hug it. As a result, Gulpin came to associate hugging with abandonment, which nearly strained its newly-formed relationship with Ash when he also hugged it. Later, it helped Clemont to break the bonds holding Ash and Volcanion together by using Gastro Acid.
After being released from its Poké Ball, Gulpin will wait until an opponent passes by, then will use Swallow to trap and constantly damage them. The opponent may also be beat on while in Gulpin's mouth for extra damage.
"A Stomach Pokémon. While it's a carefree Pokémon with a lovable face, it can swallow something the size of itself in one gulp. Its stomach is filled with gastric juices strong enough to dissolve anything it eats. As it matures, it learns Wring Out, a move which is as strong as the remaining strength of the Pokémon. It evolves into Swalot."
Gulpin, given its category, may be based on a stomach, given its round amorphous shape and the digestive gases it produces.
Gulpin may also be based on a single, giant poison gland or even a gallbladder, the green sac-like organ that stores gastric acid. It also shares similarities with some snake species known to swallow their prey whole, though its general shape resembles a slug, or also a sculpin, particularly the blobfish subspecies, known for their soft, blobby-textured carcasses upon washing up dead on shorelines.
Gulpin could be a shortening of gulping (swallowing). It may also be a combination of gulp and sculpin, as its appearance vaguely resembles the sculpin subspecies, the blobfish. It could be also a combination of gulp and in.
Gokulin may be a combination of ゴクリ gokuri (the sound of gulping) and toxin.