Demon Frog (Devil Frog in Japan) is a petite yet powerful monster ordered by King Dedede in the episode Frog Wild. The frog was immediately attacked by the king, who didn't know about its abilities. The frog is later found by Kirby floating on a lily pad in a pond by his house. Soon after encountering him, the Demon Frog possesses Kirby and causes problems all over Cappy Town. After the possessed Kirby enters the castle he starts figthing with Dedede. Dedede eventually steals the frog and uses it to posses himself. The now powered-up King Dedede begins to pummel Kirby. A monster Dedede ordered a while back, Heavy Anaconda, comes out from hiding and eats the Demon Frog, freeing Dedede from the frog's possesion.
Demon Frog is a small round frog with small bat-like wings. It has large orange eyes and purple markings all around it's body. Whenever the frog possesses people, the victim gains glowing red eyes, menacing facial expressions, and sometimes even a change in body type.
When Kirby gets possessed by the Demon frog, he greatly resembles his normal self, but he has bat wing markings on his back and his eyes get longer and get sunken in. The eyes also begin to glow dark red. He is also commonly sneering while possessed, too.
Dedede certainly changes much more than Kirby does when he's possessed by the Demon Frog. His muscles grow to epic proportions and his lower body gets leaner. His robes rip due to the dramatic size increase and he now has small bat wing-shaped markings on his chest. Like Kirby, Dedede's eyes begin glow red when possessed, but he also grows small fangs.