If you want traditional beings, rather than monsters who “kidnap” people the Huldufólk (I think the English word for it is Hulder folk, or the Hidden people) of Iceland, have been known to technically kidnap people… We do also have elves who do the same thing, and they are often considered more hostile towards humans than the Huldufólk are.
There are two ways in which they do this. The first one, and the perhaps more benign way, is that they lure shepherds or travellers who have gotten lost in a snow storm, or thick fog on the highlands into their abodes, which are usually located in cliffs, large hills, or large boulders. In some cases, they appear to humans as kindly strangers and befriend the humans, and invite the humans to their homes, and the humans don’t realise they are dealing with Huldufólk, or elves, until they are inside the homes, and the cliff/boulder/hill closes behind them, and then each day that passes inside, a year or even a decade passes in the world outside. It is VERY hard to leave again and very few actually escaped. This kind of “kidnapping” is usually not done with ill intent, but rather because the Huldufólk or elves fall in love with the human, or just befriend them, or in some cases, the human helps the Huldufólk or elf in some way, and they “repay” us humans by inviting us into their hidden world. Some humans who fall in love with one of them even choose to willingly join their world, and leave the world of the humans behind.
The less benign way is what we call the Umskiptingur, or the Changeling. The origin of this superstition very likely comes from the Irish, Welsh and Scottish roots of our ancestors, since I have heard similar versions of this from the United Kingdom.
But, the Umskiptingur is a baby… a human woman has a baby, and the baby thrives for the first few months, and in most stories, the baby is gorgeous, and such a sweet baby that it is a wonder to all who know it… but then suddenly, usually around the age of one year or so, when you’d expect the child to start trying to walk, and making sounds like the baby is imitating speech and such, the baby suddenly changes. Usually the child is much the same physically, but it’s moods and general personality does a complete 180°. The child becomes peevish and cries constantly, can’t be soothed by anything that used to soothe it, and very often starts to eat enough for a few babies, which in poorer farming homes could become detrimental to the health of the rest of the household. And very often the baby would start to eat things that were considered the “fancier” options of sustenance. Fatty foods and such.
When this happens, the way to figure out if your child was an Umskiptingur, was to leave the baby alone in a room, preferably where the baby could get its hands on something edible, or possibly even coffee, if that was available in the home. The the parents should hide out of sight, and wait, and ignore the child as it cried and wailed. Eventually, the Umskiptingur would grow annoyed, or figure that it was completely alone, and then it would shapeshift into its proper form, which very often was a grown male Huldufólk or elf, and make some kind of remark about it being something other than a baby, and walk over to where the foodstuffs were within reach. Then the parents of the baby should jump into view and either scare the Umskiptingur, or beat him, until his wife would come and save him, and bring the parents’ actual baby with her.
The most famous of these is where the Umskiptingur gave himself away, when the mother of the baby made some kind of a bizarre display in the room he was in, and left the room, and the Umskiptingur said, “Father of seven children am I in the elf home, but never have I seen anything like this before.” Then the mother of the child jumped on him and beat him, until his elf wife came to rescue him and returned the actual baby.
In some rare cases of this, the elf mother exchanges the human baby for her own irrepressible child, because the human child is so much better behave, or even more beautiful. In those instances, it’s usually necessary to beat the Umskiptingur, or deny them any kind of care until the elven mother decides that it would be better if she were to raise her elven child and return the human child.