Fire Flies always remind me of will-o'-the-wisps. These maybe considered creatures rather than humanoids, but the name and some interpretations of the phenomenon are humanoid. The latin name for this phenomenon is ignis fatuus (foolish fire.) There are will-o'-the-wisp legends (or ones with a similar glowing ball of light that shines in the dark woods and retreats when you get closer) around the globe. These ghost lights are see at night over marshes, swamps and bogs. In the stories and legends these lights are sometimes helpful, showing safe paths to follow or leading people to treasure, but mostly dangerous leading people get lost, to their deaths or to something horrific.
These lights are mentioned in literature frequently, from Shakespeare to J.K. Rowling to Tolkein and often go by other names even in English, such as jack-o'-lantern, ghost candles and hinkypunk. They can also be called hobby lanterns, note the similarity to hob-goblin. According to Hermann Hendrich (1854-1931) the wisp part of the name refers to a bundle of stick that might have been lit with fire to act as a torch or lantern.
The Will and Jack part included in some of the names seems to come from legends of individuals with those names that must carry lights in marshes for some offense to somebody. These legends can be found in Ireland, Scotland, England, Wales, Appalachia and Newfoundland.
In other European countries the lights are attributed to mischievous spirits such as Puck, puca, fairies or ghost that are trying to get people lost, or pixie-led, or could signal the nearness of a funeral.
In North America, these lights are called brujas (witches) or luces del dinero or tesoro (money or treasure lights.) The second name refers to the myth that these lights mark buried treasure places. Other names from the US are spook-lights and orbs.
In South America, the name Boi-tata is used in Brazil. It comes from the Old Tupi Language and means fiery serpent. There is an interesting legend associated with the name that I found on Wikipedia - Will-o'-the-wisp about a giant cave snake that survived the great flood and eats only eyes. The name luz mala (evil light) comes from Argentia and Uruguay.
In Asia the lights can be known as Aleya (marsh ghost-light) in Bengladesh and West Bengal, as Chir batti (ghost light) near in India, and as Hitodama (human soul) or Hi no Tama (Ball of Flame) in Japan.
In Austrailia the equivalent is known as a Min Min light. There are even myths among the Indigenous Austrailians pre-dating western settlement of the region.
Some possible remedies to protect yourself from these malicious spirits are turning your hat or coat inside out or to stick a knife into the ground blade up. [I think the first sounds safer than the second, personally.]
The myth may be based in part on Fox Fire, a naturally occurring forest bio-luminescent fungus that glows with the same chemicals a fire fly uses, luciferin and luciferous. This fungus can be found glowing among kicked up leaves, on tree trunks, etc.
The most likely source for the legends is swamp gases that also sometimes glow. There are people who have experimented to prove that the gases do exist and are capable of lighting themselves. That they can disappear upon approach and reappear elsewhere.
Regardless of the source of the legends, this sounds like a cool think to witness. According to Wikipedia - Bridgewater Triangle a place called Bridgewater Triangle in Massachusetts, USA, has had modern observations of these ghost lights. Maybe I'll go check that out someday.