Once upon a time, there was torrential rain. Such heavy downpour that the animals saw their homes flooding. They run to the hills, the flooding got worse; they run to higher hills, the flooding was still getting worse; eventually they couldn't help but gather together onto the largest hill of the region.
Such a ruckus wouldn't go unnoticed by the Mboi Guazú, the giant serpent; she woke up from her deep slumber, feeling a bit peckish. Unlike most animals she could see in the dark, and what she saw was a feast. Such abundance of prey! She could even ignore their meat, and go straight for the tastiest bits: the eyes.
So she ate the other animals' eyes. One by one. She ate so many eyes that they wouldn't fit the serpent's belly, but she kept eating them. So the eyes started appearing over her body, in-between her scales, creepily emitting light. The more eyes she ate, the more eyes she would have over her body, to the point that she was bright, she was light, she was fire.
She has become the Mboi-Tatá, or the "fire serpent". And she still roams those lands, looking for prey, burning the path as she goes through. If you ever find her while roaming, don't ever forget to close your eyes - and hope for the best.
Okay, that doesn't answer your question but I was in the mood of sharing a bit of the Guarani mythology, the fire serpent. This version of the myth is the one from the Mbyá.
If anyone wants I don't mind sharing other Guaraní myths. I also remember a few Kaingang ones.