I think, judging from the description in the Animal Ears tag, that it is intended to be used either for original characters who have animal ears (that is to say characters who are not from any known series, and thus their posession of animal ears could not be inferred simply by knowing about them from the series), or for characters who normally don't have animal ears but are being portrayed with them in that specific piece of artwork.
Now, it seems to me that if someone was going to search for the Animal Ears tag, they'd want to look through EVERY case of it, and not just the ones that don't normally have it as part of their characterization, but it seems that the people who made the tag disagree with me.