Sjoring hard-heart wants me to kill the thieves guild bosses. I can't seem to find Aengoth at the Rat in the pot in Ald Ruhn. I've been there several times but he is never there?

Did you look down the stairs which are up near the bar? If I remember correctly, he's not down the big stairs by the entrance. You have to go up that first set and then there is a small stairway to the right.