Demon Frog (Devil Frog in Japan) is a petite yet powerful monster ordered by King Dedede in the episode Frog Wild. For a creature so small, the Demon Frog is able to cause mass panic and hysteria in short amounts of time due to his ability to possess people and make them do evil things. The frog was immediately attacked by the king, who didn't know about its abilities, upon arriving at the castle. 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 fighting with Dedede. Dedede eventually steals the frog and uses it to get himself possessed, much to Escargoon's shock. The now powered-up Demon Dedede begins to pummel Kirby. That is soon interrupted when a monster Dedede ordered a while back, Heavy Anaconda, comes out from hiding. It eats the Demon Frog, freeing Dedede from the frog's possession, while swallowing him at the same time.
Demon Frog is a small round green frog with a yellow underbelly and small purple bat-like wings. It has large orange eyes and purple markings all around its 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 wings on his back (Only be seen while he's spinning with Waddle Doo) and his eyes get longer and sunken in. The eyes also begin to glow dark red. He is also commonly sneering while possessed too, making evil laughs.
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 unlike Kirby he also grows small fangs.