Legend of the Black Marauder....I really, really need to learn how to properly art, since Zerg-infested Battlemechs would be a kickass crossover project.
You've seen Infested Terran units like Marines? Scale it up to Battlemechs.
"The Black Marauder resembles an MAD-3R Marauder with flat black paint, although its shape supposedly looks somehow "wrong" and its "proportions not quite right", too sleek yet of terribly imposing bulk and with too-smooth angles. It has no serial number and no other factory markings whatsoever.
Its presence seems to grow larger when sight is obscured by darkness, fog or smoke, incidentally circumstances in which it seems to perform markedly better.
In some instances the 'Mech was—always inconclusively—described to move in ways that are impossible for a Marauder (or any 'Mech), including one tale where it was said to have turned its "head" by bending its long snout just forward of the arm joints and revealed a maw with rows of serrated teeth."