Brain Golem

Brain Golems are huge hulking golems created from animated brain tissue.

Characteristics
Brain Golems are very similar to Clay Golems in terms of immunities. They are immune to magic, cold and electricity and only partially vulnerable to fire. Most importantly, just like their clay brethren, Brain Golems resist all types of physical damage save for blunt and even then the blunt weapons must be of at least +2 enchantment or better to cause any harm.

Location
Brain Golems are generally created by Mind Flayers to guard their lairs. In two of them appear in the Mind Flayer Dungeon in Underdark guarding the Elder Brain. The latter's ability to stun party member makes them particularly dangerous.

Trivia
There is another version of the creature in the game files, GOLBRA01, which is resistant, but not immune to slashing, piercing and missile, but apparently is not used anywhere in the game.