The cueball is an "ambient" monster in Doom Eternal, a zombie fused with gas canisters similar to the possessed engineer from the previous game. With very little mobility and no attack capability, but exploding violently when killed, they are functionally closer to explosive barrels than to actual enemies. When hit, they are pushed back strongly and then explode.

Just like the cueball in billiards is used to hit the other balls, this monster should be sent toward other monsters to damage them.