Arachnotron

The arachnotron is a monster introduced in Doom II. It is a miniature Spider Mastermind that, instead of bullets, fires green plasma shots. When an Arachnotron begins firing, it will not stop until either its target moves out of its view or dies, it is stunned (causing it to move) or it is killed. Similar behavior can be observed with the Chaingunner and Spider Mastermind.

According to the manual:
 * Maybe cybernetics wasn't such a great idea after all. Look what the demons have done with it. It seems unfair somehow you're not the only guy in Hell with a plasma gun.