Swalot, the Poison Bag Pokémon. Swalot has a body that can expand and contract at will, and can swallow anything whole.
– Ash’s Pokedex
I’m not sure what Swalot is supposed to be. Its species is “Poison bag”, so I guess it’s a bag of poison? Its whiskers remind me of a catfish.
Koga from HGSS’s elite 4 has a Swalot on his rebattle team. I remember the first time I fought it, and didn’t have a Psychic or Ground move (and my Pokemon were underleveled), and it took me forever to defeat it.
Stat-wise, it has higher defenses and HP than attacking stats, which makes it a decent defensive Pokemon I guess.