A Tulpa is a thoughtform, or being created from the collective thoughts of separate individuals. The concept of Tulpas is theoretical in nature and originates from Tibetan mythology, where Tulpas are described as extra bodies that were created from one person's mind in order to travel to spiritual realms. The Tulpa Effect is the name given to the unintentional creation of a Tulpa based on collective belief of a being with similar traits.
In relation to Slender Man, the idea of intentional creation of Slender Man as a tulpa came as early as August of 2009, as the idea was first thrown around in the Something Awful forums. The idea is still carried today by some Slender Man-based communities, specifically Slender Nation, although the various separate evolutions of the Slender Man creature have created varying beliefs, traits, descriptions, and actions, and no longer includes a single unified description of Slender Man from which a tulpa could be created.
Core Theory centers around the idea that the Slender Man was created as a tulpa in some way in the various ARGs, and acts as an in-game reasoning as to its existence and slight differences among the various series.
Some have worries over the unintentional creation of a Slender Woman tulpa due to the popularity of the mythos in present day. Others believe that the Tulpa Effect may have serious real-life repercussions with the creation of a real Slender Man.
"Tulpa is a concept in mysticism of a being or object which is created through sheer spiritual or mental discipline alone." - Wikipedia
A competing theory of Slender Man's origin is Quantum Theory.