Now now, you guys. Carbuncles predate any version of Final Fantasy by, well, at the very least quite a bit.
A small animal alleged to have been seen in 16th Century Latin America by Spanish Conquistadors. Legends say the Carbuncle has some sort of shining mirror or jewel on its forehead, reminiscent of the gemstone by that name. Many explorers supposedly searched for it, hoping to secure the precious stone—but without success.
I'll admit I only know of the name from Final Fantasy in the first place myself, but with series like Ancient Magus' Bride digging up all kinds of different critters from different mythologies all the time I've been looking up more and more about all kinds of things lately. The more I look into it all, the more I realize writers really haven't been coming up with very much new stuff to put in their settings over all, which is fair to be honest. With the sheer size and amounts of the currently existing collections of myths, you really can't run out of things to work with and a lot of them are things most people haven't even heard of anymore 'cause the folklore stopped passing down as much as belief in it fell.