The Steadfast Dwarven Spider is a Dwarven Automaton that can be found in Kagrumez. After completing the first Kagrumez trial, proceed through the newly-opened gate to reach a room with a bunch of random Dwemer things in it. You will notice a lever; hit it to open the wall, and you'll find the Steadfast Dwarven Spider. It won't attack you, and if you talk to it you can make it your follower. Since it is a creature rather than an NPC, it cannot carry items for you as a follower and it cannot be your follower if you have a dog.