Actually several summoning-class skills have no requirement for Summoner, but usually because you don't have a choice about who can learn them.
Treated Like Cattle is somewhat bugged in that the key to ALice's cellar is found on her body, but if you kill her, you can't complete the quest. You need to lockpick the cellar.
Mordus's amulet is also a key, as well as a quest item. Lone Wolf Mark is also a quest item, I believe.
