The Brain Bug is a large spider-like creature.
Brain bugs will walk towards Albert West and spit a slow-moving projectile when in range. This acid explodes upon impact and slows West down for a second. Brain bugs always spawn with two Small Spiders, although they occasionally get separated. When the brain bug dies, its two small spiders will die soon after.
Killing the small spiders leaves the brain bug harmless since its acid projectile doesn't do anything but slow West down. When it's alone, any weapon will suffice for killing it. Since the small spiders die with the brain bug, however, it is possible to try avoiding the spiders while aiming for the brain bug. A shotgun or melee weapon can be useful for this, since the constant moving can make aiming with a pistol or SMG difficult.