From Non Uberis
https://www.fimfiction.net/group/212335/monster-mares/thread/503905/cavalum-the-portuguese-horse-bat-devil#comment/7748079

While reading something a friend wrote, I saw the word "cavalum." As I try to do whenever I see a distinct word I don't recognize, I looked it up, and I discovered something interesting.

In the folklore of the Portuguese island Madeira, the Cavalum was a demon (or literally just an incarnation of the Devil) which took the form of a horse with bat wings, capable of breathing fire and controlling storms/the weather. One day he entered a church to challenge God and threw a hissy fit, surrounding the church and island with a storm until God had enough of his shit and sealed him in a cave. This ties into the Furnas do Cavalum, a series of lava tubes on the island; it is said that you can hear the struggling of the Cavalum within the caves during a storm, and that it will devour any who wander inside.

There is also a legend of a similar creature, the Cavalo de Pensamento ("Thought Horse"), which has similar traits and abilities but is of a more benevolent nature. It is said to appear to those who think of it and offer them a ride to wherever they need to go. Because it has the same power as the Cavalum, though, it would often cause freak disasters and mass destruction by accident.

Why do I bring this up? Well admittedly mostly because I think it would be a lot better for us to start referring to batponies as cavalum than thestrals. From what I have been able to glean, thestral is a name and concept wholly made up for the Harry Potter series, and I'd much rather use something that has true mythological origins. I have seen that this notion has been brought up in the past elsewhere on Fimfiction.

But also, if the idea of a proper Cavalum pony who burps fire and whips up storms and dethrones God is something that appeals to you, go nuts.