Reflections can act as the link between worlds. Though this is rare and takes special magic.
However dreams can also link planes of existence.

This is more common and, unfortunately, happens all by itself.

Since Princess Luna's return creatures from another world and their fantasies have been crossing over to and polluting the Equestrian dreamscape. To combat this Princess Luna directed that her Lunar Guard create the Department for Dreamscape Defence.

This is one tale of their activities.

A Human in Equestria story (with a difference), because I wanted to write a one off.