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 grassy areas.
A Gulpin appeared as one of the Pokémon living at the Nebel Plateau in Volcanion and the Mechanical Marvel. 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's appearance may 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.
Gulpin is a shortening of gulping (swallowing). It may also be a combination of "gulp" and "sculpin", as its appearance vaguely resembles the related blobfish.
Gokulin may be a combination of ゴクリ gokuri (the sound of gulping) and toxin.