Droppy is an enemy from Kirby & the Amazing Mirror. It is a small, yellow creature that can inhale and swallow like Kirby, and it also has its own method of copying abilities. If Kirby gets too close to a Droppy, it steals the copy ability that Kirby currently has, then transforms into a faster and pink version of an enemy that gives that power when Kirby swallows it.
By eating the transformed Droppy, Kirby gets the copy ability that the Droppy stole back. Also note that Droppys that haven't transformed carry no ability.
If Droppy steals a mini-boss copy ability, like Hammer, or Master, it won't transform into that mini-boss (or transform into a completely different enemy at all). Instead, it will simply change its color to red and start running around.