Cacodemons demons from the Doom series with large spherical bodies crowned with horns, that float slowly through the air. They have a single green eye over a large mouth which is persistently twisted into an evil grin showing a series of nasty teeth and a bluish interior.
In Doom II, their instruction manual description says: They float in the air, belch ball-lightning, and boast one Hell of a big mouth. You're toast if you get too close to these monstrosities.
They attack by spitting a ball of lightning or by biting with their gaping maw. The cacodemon often seems to distance itself from the player when under attack, though the cause of this is that it is being pushed back by successful hits from its enemies.
Cacodemons emit a loud hissing sound when they first see the player or otherwise become active, and use the pain sound shared by most of the demonic monsters from Doom. When killed, they let out a gurgling moan and fall to the ground, splitting open like a burst pustule, spewing copious amounts of blue goo and intestines that flow forth, to reveal their bared brains while their sole eye pops out of its socket.
Cacodemon attacks are relatively easy to dodge, and with sufficient space large numbers of cacodemons can be fought while sustaining little or no injury. Because they are slow, and because their pain chance is high, they are fairly easy targets. The chainsaw can incapacitate them, and repeating weapons such as the chaingun or the plasma gun are very effective against them, as they are pushed away and are nearly unable to fight back. They are relatively easy to goad into attacking other monsters, but can occasionally be tricky in this respect if they are floating in a high position allowing most of their shots to go over other monsters in the player's area. A skilled player could use a shotgun, run in for close shots, and dodge any incoming attacks while reloading and retreating.
Because of their flight ability and versatility, it is not uncommon to see these flying out of unexpected angles once alerted.