In the horror movies done in the human world, some foolish human summons a demon or devil from the hells. In truth while it is possible for a human to summon a demon or devil from the hells it also possible to summon any mythical creature from their home plane which exists outside of the human world. With the right combination of spells, glyphs and the like a person could summon anything from a dragon to a fairy.
While it is possible for a human to do that, that is very common and the magic/methods to do so have all but been lost to humans…. but not the other mythical races.
Across the human world every day people disappear, be it in singles or groups and are never seen again. Most humans believe this to be either the human running away or another human murdering them. Only a few crazy people believe the truth, that they were taken not by humans but by magical creatures.
When they are transported, sometimes at random sometimes even targeted to a new world where they will be transformed into the creatures who call that world home. When they are transformed everything can change, an old man can become a young female fairy or become a dragon's egg waiting to hatch. A women can become a mermaid as easily as they become a merman or a scaled covered naga. Anything is possible.
They do this as payback for when humans would transport them, taking human blood as payback for when humans took them. Some take humans to transform them into second class citizens in their world, some take humans to repopulate their race after what humans did to them, some do it to find a mate, and others do it because they can.