Boggart

A Boggart in the Harry Potter fictional books is a shape-shifter that takes on the form of its intended victim's worst fear. It generally likes to hide in dark, enclosed places, such as in closets, under beds, or in hollow trees.

Wikipedia contains spoilers

In Harry Potter and the Prisoner of Azkaban, Professor Lupin teaches his students in Defence Against the Dark Arts to approach a boggart in groups of two or more, so that the boggart will have difficulty in choosing which one to frighten and also so that if one targeted is frightened, the others will be less so since they will likely have different fears. If the boggart can be tricked into taking on the form of two different creatures at once, the effect will likely be to make the boggart look ridiculous rather than frightening; at that point, the boggart can be vanquished by saying (with wand pointed), Riddikulus!. Lupin later uses a boggart to teach Harry to fight Dementors, as they are Harry's greatest fear.

Also in Harry Potter and the Order of the Phoenix, a boggart is hidden in a drawer in Number 12 Grimmauld Place. Mad Eye Moody confirms its existence with his electric eye. Mrs. Weasley tries to get rid of the boggart. Harry approaches to see the boggart shifting into dead bodies of the Weasley family and Harry Potter himself. The others however approach and get rid of it.

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In Celtic mythology, a boggart (or bogan or boggle) is a household spirit, sometimes mischievous, sometimes helpful.