When doors between worlds are opened, everyone is put in great risk. The act of opening such doorways not only connects two worlds (or dimensions or universes or timelines), but also weaken the barrier between worlds. Creatures, both benelovent and malicious, can not only travel between two connected worlds, but also break through weakened barriers to arrive in a non-connected world.
 
Such is the case when the Cutie Mark Crusaders go looking for their friend Silver Spoon in the Everfree Forest. Few places in Equestria has a weaker barrier than the Everfree Forest, resulting in magic and physics as understood by ponies do not work as elsewhere. Due to the meddling of other worlds weakening the barrier more than usual, a malovent creature was able to rip itself into this world. A creature that is known another group, not so different from the CMCs, would refer to as the Demogorgon.
 
Just some MLP and Stranger Things fan art. For maximum experience, I suggest listening to Follow You Down by Aviators ft. 4everfreebrony.
 
(11” x 14”)