Saw this while strolling through posts, and the first thought that came to mind was "why?". We already have "ponytail", and as far as I know there isn't any tagging for a pony tail (as in the tail of a pony) like with other animal-specific tails (which often get applied to anthro animals even if that's not what the tags are for, but that's another issue).
Is there any reason to keep "ponytail_(hair)" as a separate tag, or just merge it into "ponytail" (manual editing or alias, there's only 26 pictures so tagged as of this post)?