Those are Dragon Priest masks. As for Yngol's helmet, I don't know. The wooden mask is found on the corpse of a bandit/bodyguard in the area surrounding Labrynthian along with some interesting notes. The guy who the mask belong to (according to the notes) put the mask on in the area of Labrynthian and disappeared, he did this a few times. I think maybe that is a clue to what it does but i have not bothered to test this myself. It has no apparant use inside Labrynthian (i was there earlier for a quest, took the mask with me) but maybe it does something if used in the area directly outside the dungeon. If u equip the wooden mask right where u find it u get transported to a room with abutch of the dragon priest statues where u can store the mask althouygh i dont no what happens when u put all of them on the statues( there r u few empty chests)