The Demon category is one of the two that aren't recognized in the US enemy chart and NP strategy guide. Since there isn't any official source on it, this category even being "Demon" is just my guess. I got the idea looking thru the categories in FF9, and it seemed to make sense.
This category is frickin' huge considering the only significance of it is that the Rune Sword and Excalibur have a greater effect on them. This of course only applies to the WSC/PS versions.