Popo

Popo is a one-time enemy in Super Luigi Country, only appearing in Level 14. He is a main character in the Ice Climber series. He is equipped with a hammer.

Story
Popo's sprite in-game is bigger than normal, so it could be implied that Popo ate a Super Mushroom before encountering Luigi. It is unknown how Popo acquired the Mushroom.

Level 14
In this level, Popo stands near the far left of the middle platform guarding an Eggplant with his hammer. Somehow, he doesn't squish it.

Trivia

 * Popo is usually accompanied by Nana, but she is no where to be found here.