I would like to give it a shot. Because this alias request will certainly come up again in the future.
Firstly, I would like to propose to use completely separate tags to describe the number of horns and the kind of horns.
If a girl has a single red eye, we currently tag her with cyclops and red_eyes. So no overly specific tags like one_red_eye or single_red_eye.
In the same vain, I think we should drop singular horn tags like demon_horn and oni_horn.
Also because I think that using oni_horns to describe those kind of horns is not-completely-perfect to begin with, but more on that later.
Number of horns
Looking back on the previous topics, people really want to differentiate between a single horn and multiple horns.
By separating the two, we can ignore the specific kind of horns for now, and just look at the number of horns. Some possible examples are:
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Or whichever tags people would prefer.
Now @Schrobby mentioned here that we could also split-up posts with two horns and posts with three or more horns.
Schrobby said:
True. While we're at it we could also rename horn to single_horn and add multiple_horns for instances such as post #2055810.
Which would be fairly easy to implement as well.
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Now because this would create a lot more tags describing characters with horns, it might be appropriate to create an umbrella tag describing a character with horns regardless of number. Something like horned or horned_character for example.
The kind of horns
Now the messy counting of horns stuff is out of the way, dealing with the specific kind of horns is fairly easy. Like how we deal with eye colors, just use the plural everywhere like we currently are.
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Which brings us back to oni_horns
This tag is currently used for two kind of horns:
Splitting these up can be done in various ways, though I'm not sure which is the best way.
Comments, feedback, insults are all very much appreciated.
Sidenote: About Ibuiki Suika; she is canonically an oni, and she has horns, but would you call her horns oni horns? They are nowhere near her forehead.