now, let's imagine that from now on only canonical animal forms qualify, what should be done, should all the above mentioned tags be nuked because are no canonical/are fan made?
Pretty much, yeah. The only downside I can see is that older characters can have their search slightly muddied from the sheer amount of artworks they appear in, but i think it's better than to maintain some 40 character tags, most of them single-digit, which can all be grouped up under a single gentag.
under this criteria, the fact that a character is based on a mythological animal does not mean that it is canonically a animal form.
furthermore, if we say that the latter DOES count, then toutetsu would qualify under this criteria, as we know, she is a taotie, not an ordinary sheep.
so, once again, it should be changed sheep to taotie, just look at urumi as example, it says "ushi-oni" not "cow".
If we really want to nitpick, Mystia is not supposed to be a generic bird, but rather a Yosuzume, but because the original translation decided to translate Yosuzume into Night Sparrow, english-speaking people preferred to leave it that way (not like most people know about it anyways). Her tag is not Mystia Lorelei (yosuzume) nor Mystia Lorelei (sparrow).
It wouldn't make sense to change it to either though, because sometimes artists take artistic freedoms - for example, Fujiwara no Mokou (phoenix) cannot be used in post #1606691, because she's depicted as a cockatoo. Would we have to make a Fujiwara no Mokou (Cockatoo) tag? Or just tag it as animalization? But then, why not directly alias Fujiwara no Mokou (Phoenix) to Fujiwara no Mokou (bird), to make up for these fringe cases?
I'd like to go with the principle of Occam's Razor, and group all of them under animalization. It's much simpler than to figure out where arbitrary exceptions should be made and where not.
(As a side note, Meiling (Panda) is also arbitrary, she's not based on a panda, she's just chinese.)