Boggarts are a fictional species of spirits seen in the Harry Potter series.
Boggarts usually haunt abandoned places. When a sentient creature approaches the Boggart's lair, it will shapeshift into its opponent's worst fear, which, if the being does not recognize the Boggart for what it really is, will send it away. Boggarts that turn into magic-users cannot truly use magic and only give an illusion of it, or, at best, a weaker approximation; however, a Boggart transformed into a physically imposing form does possess the physical strength of the creature it impersonated, making large Boggarts very dangerous.
Boggarts can be forced into a different form with magic; the most common way to achieve this is to use the spell Ridikulus, which turns the Boggart into a comedic version of whatever form it was using beforehand. A Ridikulussed boggart will quickly retreat, realizing it no longer scares its target.
Long-lived Boggarts will occasionally come into a form they particularly like, usually one that scares every witness equally. The extent of Boggarts' intelligence is unknown, although there is one recorded case of one such long-lived Boggart impersonating a murderous human thug for such an extended period of time that at least some level of sapience must have been required to keep up the charade.