The damn thing outsmarts me... I finished the game last night and without being afraid of spoilers went to search pics of it. I found out there's one different looking mob walking around too. I didn't even know there was different ones since this is the only one I've acually seen. Haha.
Game: Amnesia: the Dark Descent
Edit: I KNOW it's brute in the sewers. While playing this for the first time I didn't stop to stare the monsters, I just ran like a little animal.