The Imp is a newcomer in Zombiecraft 3. These enemies will replace the hellsheep. Just like hellsheep, imps have less health than zombies.
The Imps move around similar to zombies, except that they keep a distance from you. Once close enough, the imps will shoot fireballs at you, which can get you down to half your health with one hit (without iron chest armor from juggernog).
Imps seem to come around every 10 waves, and sometimes make an appearance the wave after. It is unknown the algorithm behind imp spawning.
The Imp is a reference to the video game, Doom.