I think the problem with all the demon tags is that depictions of demons tend to be very varied depending on culture, genre, and personal taste. A demon can be a big red humanoid with horns, a winged humanoid with a goat's head, horrific abominations, weird animals, cute girls, randomly generated monstrosities represented by '&', etc. It's a pretty vague tag as a result.
More so for the demon_girl and demon_boy tag. The criteria most people are using are probably leathery wings and horns, which can also apply to tags like dragon_girl. It's even more troublesome when it's not a full body view; is that supposed to be demon_horns or goat_horns or just plain old nondescript horns? Is that a black dragon_wings or demon_wings or bat_wings? Also, is there a border between, say, being a demon_boy and plain old demon?
Also demon_wings and bat_wings tend to take artistic liberties so it's hard to tell when one is supposed to be the other since they're both visible as leathery wings. Same with dragon_wings. Just look at some remilia_scarlet and koakuma and say which wing is supposed to be which.
Just my two cents on the thing.