Dokkaebi are generally identified as Korean goblins. Information I’ve found about them online generally makes clear that this doesn’t necessarily mean they’re evil, but then, neither are European goblins. There are many different sorts of goblins, just as there are fairies, elves, and imps. Dokkaebi are often portrayed as mischievous pranksters, but they can also be benevolent at times. They’re blamed for causing sickness, but there are also rituals to ask the goblins for good fortune. Many stories run with the idea that they help kind people and trick greedy ones. One has a man sell a lump on his neck to a dokkaebi by convincing the creature it was what allowed him to sing. Another man hears about this and tries to do the same, but only ends up with a second lump. In a different tale, a boy manages to steal a magic club from the dokkaebi by scaring them away with a walnut, but they aren’t fooled by a greedy boy who follows suit. There’s also one about an old man who manages to trick a dokkaebi into believing his greatest fear is a lot of money. It’s apparently common as well for dokkaebi stories to involve wrestling matches, specifically ssireum, in which contestants try to pull opponents down by their belts.
Sometimes the wrestling dokkaebi have only one leg, making it easy to defeat them if you’re aware of this. The creatures are said to prefer darkness, although there are some who will come out in the daylight. They’re known to possess two major sorts of magic items, the dokkaebi gamtu, a hat that makes its wearer invisible; and the dokkaebi bangmangi, a club that can produce pretty much anything, sort of like the magic hammers carried by some Japanese oni.
I’ve seen it mentioned a few places that dokkaebi are formed when inanimate objects gain spirits, sometimes because they are stained with human blood. I don’t know that ALL dokkaebi have that origin, but it seems to be a popular one. It’s said that the first written mention of dokkaebi is in the Samguk Yusa, or Memorabilia of the Three Kingdoms. There’s a story about King Jinji of Silla, one of the Three Kingdoms of Korea, who ruled in the late sixth century, propositioning a peasant to be his concubine. She refuses because she’s married, but consents to going through with it if her husband dies. He does, but not until after Jinji dies. Regardless, the king shows up as a ghost to have sex with the woman, and she gives birth to a child named Bihyeong, who talks to ghosts and dokkaebi. He has them build a bridge over the Hwangcheon River. Much more recently, there was a television drama with Dokkaebi in the title, telling the story of a goblin who wants to find the woman who will end his immortality if she marries him. From what I’ve read, it doesn’t have that much to do with traditional dokkaebi lore, and the idea of becoming human by marrying someone seems more like Christian stories of magical beings where marrying a mortal will give them a soul. And a fair number of Google search results refer to a Tom Clancy character with the call sign Dokkaebi. The reason I looked up dokkaebi was that one appears in Yoon Ha Lee’s Dragon Pearl.