Outsider

The Outsiders are a kind of very powerful beings.

Description
Little is known about them. The Outsiders had been the servants and foot soldiers of the Old Ones.

Outsiders can only be called up by mortal magic.

The White Council's Seventh Law of Magic forbids the contacting of Outsiders.

Outsiders are immune to most forms of magic.(ref)

The Walkers are Outsiders and include He Who Walks Behind and He Who Walks Before. (ref)

Known Outsiders

 * He Who Walks Behind
 * He Who Walks Before

In the series
In Dead Beat, during The War, the Red Court had called up Outsiders against the Council, who were retreated wiht wounded, along the Ways in Faerie.

In Cold Days, when discussing He Who Walks Before, Mac tells Harry there are three of them, Who Walk Before and two others. This may mean there are three Walkers in total, He Who Walks Before, He Who Walks Behind and one other.