Shellos Pokemon GO was temporarily available for a day before being pulled by Niantic Labs.
It's unknown why the Pokemon was added for such a short amount of time, but some speculate that the regions were glitched.
Shellos Pokemon GO: Shellos Temporarily Available World Wide Before Being Pulled
Shellos is a Water-type Pokemon that resembles a slug and eventually evolves into the Water/Ground-type Pokemon Gastrodon. The Pokemon has two forms, pink "West Sea" form and the blue "East Sea" form.
After a short period of time, the Pokemon suddenly disappeared from the game. It seems Niantic either didn't mean to release it so soon or that it didn't mean to have both forms available at the start.
Niantic hasn't announced whether the Pokemon will return at a later time.
Photo courtesy of Niantic Labs