Demon Beasts

From Kirby - Wiki on Neoseeker

Demon Beasts (Called Monsters in the Kids English Dub) are a variety of dangerous and evil creatures created by Nightmare. The greedy King Dedede often buys them from Nightmare Enterprises by way of Customer Service. But each time, they're defeated by Kirby as the pink warrior grows stronger. Most Demon Beasts are created specifically for Kirby: Right Back at Ya!, but a few appear in games. For example, Kracko is a reoccurring boss in the Kirby series, and also one of the first monsters.

Though it wouldn't seem so, Demon Beasts are not evil when they're created. Some are simply residents of Dream Land, when Nightmare kidnaps them and turns them into evil monsters. They go through a specific training program to become strong and evil.