A tailhole is its own entity, separate from an anus. The distinction is that the former is placed at the base of, or directly attached to the tail like so:

https://e621.net/posts/3772654
https://e621.net/posts/3740879
https://e621.net/posts/3511102
https://e621.net/posts/3734418
https://e621.net/posts/3788033

As seen, the anus in these photos is either directly connected to or just barely beneath their tail.

I'm making this request because I rather like seeing these sorts of butts, and would appreciate if they had their own separate tag to indicate their difference to regular anuses

Tailhole is a real-world synonym for anus. I don't think it's reliably used as a synonym for "anus that's particularly close to the base of the tail".

The tag you're looking for is tail_anus.

degeneratekumquat said:
A tailhole is its own entity, separate from an anus.

Unfortunately, it has become a slang term for people who don't want to vulgarly say "asshole" or "anus".
And like what @Vulpes_Artifex has already said, the correct term you should be looking for is tail_anus.