i was scared of this one as a child. its mouth freaked me out

Gulpin is a pack-hunting omnivorous Pokemon famous for its incredibly corrosive stomach acid.
Most of Gulpin's body is taken by its oversized stomach which, thanks to its thick lining, can stretch to about 4 times its size. Its brain is tiny, and instead of a heart, blood circulates through its body freely, and respiration is done through the skin and transported through the body with tracheae, similar to insects. The antenna on the top of Gulpin allows it to smell its prey before it sees it.
Its digestive system consists entirely of its stomach, which is directly connected to its lips and anus at the tip of its "tail". The lining is full of acid-creating cells, which make sure it never runs out of stomach acid. The acid is corrosive enough to melt through titanium within a matter of minutes. This, along with its elastic stomach lining and skin, give Gulpin the ability to swallow a full-sized human being and digest the entire thing. Nutrients from its food are directly absorbed through the stomach, and the rest keeps getting corroded into nothing but a couple of atoms floating about. Its anus is used to drain excess fluids.
Due to Gulpin's lack of organs, it reproduces by budding. A small Gulpin develops on the skin, and breaks off.