There are a lot of pictures on e621 which contain Tauren but clearly contain no Bovines, but will still turn up in a search of "Bovine" because of the Tauren->Bovine implication.
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Now, I'm not entirely certain what the solution to this is, as it seems to me that this is more an issue with the shapeshifting than the species. In an ordinary image in which one character has shapeshifted to another species, the original species simply isn't tagged at all: tag what you see. The Tauren druid forms, however, are so characteristic of them that people familiar with the game can tell that they are Tauren even in their alternate forms.
Thus we have two possible solutions: either remove the Tauren->Bovine implication (Ruling: a Tauren is not always a Bovine) or stop tagging Tauren in shapeshifted forms as Tauren (Ruling: gender style "tag what you see without outside knowledge").
I'm leaning towards option 1, personally. I'd rather not have cat-form and bear-form Tauren showing up in my bovine searches, and neither would anyone unfamiliar with the Warcraft universe. (Hey, don't look at me like that!) But on the other hand, if someone else wanted to look up the Tauren cat form, it makes sense for that to show up in a search of "Tauren Feline."
Updated by Riversyde