This is a Brazilian version of the myth that explains the Will o' Wisp or Saint-Elmo's Fire which exists in almost all cultures. In Germany, it is the Irrlicht (the mad light) carried by tiny and invisible dwarves. In England it is the Jack Lantern who, in the form of a ghost, guides the travelers through the bogs and wetlands; in France it is the Sinister Moine des Marais (monk from the wetlands), with the same attributes as the swamp guides; in Portugal they are the alminhas (little souls), the souls of the pagan boys or the apparition of a soul who left behind some buried money and can't save itself until this treasure is found by someone.
In Brazil this is one of the oldest myths, and it's origin is almost entirely indigenous. It would be a fire-snake that wandered the fields, protecting against those who try to set fire to them. Sometimes it's transformed into a thick burning log that kills by combustion those who burn the fields uselessly. The boitatá was mentioned by the priest Anchieta in his São Vicente letter dated of May 31, 1560. The priest translated its name as a ‘thing of fire, that is all fire'. Mbai means thing, and tatá fire, giving the exact version: a living fire that moves, leaving a bright track. Since there's another similar tupi word, mboi, that means snake, we come to mboi-tatá, the fire snake. It is also known as a serpent of fire that lives in the water, or a big snake that kills the animals and eats their eyes; that's why it is so full of light — it's the light of their eyes. [It can be] a bull or an ox that breathes fire from it's mouth or a bad person's spirit who wanders the land, torching the fields or running around as a firecracker or a torch, in differing versions. It is known by different names in different regions of Brazil.
In the Northeast is called batatão, in the center-south is boitatá, bitatá, batatá and baitatá. In the state of Minas Gerais, it's known as batatal, and in Bahia it's called biatatá. Cautiously, the priest Anchieta said: ‘What it may be is still not known for sure'.” [link to globalvoicesonline.org