During M10: Black Hole and M15: Mother Lode, they appear as distractions. They have a simian, knuckle-walking gait, and can jump into the air to unleash a devastating shockwave when they land. They are very difficult to kill: only their joints and back, marked by green, are vulnerable. A Proximity Bomb placed at their feet will destroy them instantly, however.
Elite Bruisers are more powerful versions of this enemy, and appear in later missions.