Will-o-Wisp

A Will-o-Wisp (known by many, many variations of that name, such as Will o' the Wisp) is a strange monster that takes the form of a glowing orb of light that tends to appear in dark forests or near swampy regions. Whilst sometimes portrayed as merely mischievous, more often the will-o-wisp is a malign trickster who uses curiosity (and perhaps a dab of mind-control magic) to lure victims into following it, invariably delivering them to danger in the form of a natural hazard or a monster.

Depending on the setting, will-o-wisps may be portrayed as a kind of ghost, an elemental, a fairy, or some sort of aberration.

Fun fact: There really is a Real World phenomena that inspired the common folkloric "Will-o-wisp"; known as "ignis fatuus" (translation: "Fool's Fire" and not in fact a faux-latin insult for an obese pyromanic). The modern scientific explanation is that certain kinds of swamps bubble up various flammable gasses that ignite upon coming into contact with the air.

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  1e
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  AD&D Monster Card
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  2e
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  3e
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  4e
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  5e
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  Pathfinder's Will-o-Wisps have skulls.
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  PF 2e

Boggart

According to Monster Manual 2 from 1st edition, juvenile will-o-wisps are known as boggarts and can shapeshift into a humanoid form (a power they lose when they grow up).

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Ravenloft Variants

AD&D 2e's third Ravenloft Monstrous Compendium introduced 4 different Will-o-Wisp variants: Will-o-Dawn, which helps those who are lost or in pain; Will-o-Deep, which only appears underground and causes people to become lost and in pain; Will-o-Mist, which only appears on the misty borders of the Demiplane of Dread; and Will-o-Sea, the obligatory aquatic subspecies which is shaped like a ribbon instead of a ball.

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  Will-o-Dawn
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  Will-o-Deep
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  Will-o-Mist
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  Will-o-Sea

Hound of Law

In The Rod of Seven Parts, the Vaati/Wind Dukes of Aaqa breed a special type of will-o-wisp called a Hound of Law. These can polymorph into several different animals, but their preferred form is that of a dog.

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Ghostlight

Monstrous Compendium: Planescape Appendix 2 introduced the Ghostlight, a more powerful relative of the Will-o-wisp. They have hypnotic pattern, dominate, and a melee attack that drains levels.