Wings and tails are common sci-fi/fantasy topics, especially for humanoids. A recent top new question without a 'tails' tag made me realize I'd subscribe or watch such a tag if it existed.
I checked several other questions as well, and they seem to generally use less specific tags such as 'creature design', 'anatomy', or 'biology'.
Should there be a 'tails' tag? When the question is specifically about such a common/popular addition, should there be a tag to support it?
I think it's a matter of practicality. Pick up any subject and you can always break it into smaller subjects. We settle for scopes that are practical for the site.
We can always use anatomy for body parts. If we follow the tail idea because a question asks about tails, one day we might have questions tagged with prehensile-tail, long-tail, tail-weapon etc.
Just the same, we could take the anatomy tag and branch it out all the way into limb, lower-limb, foot up until metatarsus.
anatomy serves as a nice catch-all that anyone interested in anatomy in general can follow, and which also serves to determine question scopes in a fine enough way.
To assess the utility of a tag, let me rephrase your question: would you say that you are an expert in anatomy, creature design, biology?
Yes, and to a listener it would be clear what you excel in.
Now, let's try with tails. What does it mean you are an expert in tails? Or fingers? Or wings?
My opinion is that it is too detailed of a tag to be useful.
