Two tags for the same thing. Used quite rarely as well. Personally I wouldn't care which is displayed and which is linked to the other when used while tagging.

I'd say "Devil_tail" is more accurate.

I vote for devil_tail, since demon is usually used for a wider variety of creatures than devil is, and the tail is related to that specific thing usually found under the devil tag.

devil_tail would seem at odds with demon girl though, which is the most popular of these tags.

I also prefer demon_tail. "The Devil" just conjures up images of that big red guy from South Park for me.

+1 for demon_tail

Looking at the posts tagged with both these tags, I'd have to say devil_tail. This particular tail isn't of any general demon, it's of the stereotypical christian devil, thin with a heart shaped tip. To us there isn't much difference but if you consider demons beyond the christian mystic you'll have to include for example the japanese youkai and oni.
http://en.wikipedia.org/wiki/Category:Demons
It seems to make sense to tag it as demon_tail as to fit better with the demon_girl tag but looking at images tagged with demon_girl, it seems to be mostly succubus which though unrelated (Morrigan doesn't have a tail) seem to go hand in hand with generic demon girls these days, however this tag also includes post like this one which is a pair of oni girls. I don't think I have to remind you how important oni girls are.

But what about something like post #210318? I still think it's better if all those kinds of tails are covered by the same tag, and demon_tail makes more sense to me.

Many demons don't even have tails.

Mmmm... not many people are going to use an un-moe tail like post #210318 so I changed my mind, I say demon_tail.

All this tail business is usually in the context of girl appendages, no one is going to tag and actual demon with demon_tail or demon_horns or demon_wings, just horns, wings, not even tail. Or an actual rabbit with rabbit_ears. Because it's redundant.

By the way, I don't tag oni girls as demon girl and don't think they should be. I think of demon girls as looking like astaroth or koakuma, oni look pretty different (particularly Touhou's.) And they don't come from hell, AFAIK.

7HS said:
By the way, I don't tag oni girls as demon girl and don't think they should be. I think of demon girls as looking like astaroth or koakuma, oni look pretty different (particularly Touhou's.) And they don't come from hell, AFAIK.

Hell is a European concept. You're thinking of succubus girls or variations of them. Demon has come to be an umbrella term and is now a term used to refer to many beings in various mythologies. Check out this page:
http://en.wikipedia.org/wiki/List_of_demons
Oni, youkai, tengu and even kitsune are considered demons.
But I understand where you're coming from, we're westerners, to most of us when someone says demon we immediately think of the christian devil, the demon from OUR folklore. But the idea of a demon to the average japanese may be an obake.
That's why it's important to differentiate between them.

demon_tail seems to be better for a more broad tagging while devil_tail is faster linked to the heart-shaped end of the tail (to westerners at least)
Anyways, I made up my mind and go with demon_tail.