I've looked at all posts with the tag "pig," and they do contain a pig. Turning the tag into an artist tag must be the mistake of the tagger. That has been already fixed.
This url was under the info for the pig tag http://www.eonet.ne.jp/~happy-water/illust/MHP2G_01.jpg, though whatever it was it's not there anymore. Probably for this one image the uploader changed the tag to an artist tag, another person came along saw that there weren't any pigs in it and removed the tag.