What to do with a unicorn?
Riding unicorns is preposterous. Seriously, it always feels like a dumb idea--the image of it. Its overkill--you have a sword and your horse has a horn? What the fuck? The unicorn needs to be its own thing. Maybe Satan could ride a unicorn, that's it--maybe.
The-unicorn-is-secretly-someone-else. Ok, who?
-Someone good? Seems boring. If the unicorn is someone good then they're basically the same as they were only a unicorn--they "mean" the same thing either way, and act the same. But then maybe if the unicorn acts like an animal then maybe once you get the unicorn you have to let it go because it's secretly someone good. That's tragic. Though in D&D, seriously, who gives a fuck about some NPC? Plus what's the point of the pre-transformation unicorn-having part of the story--to temporarily have a magic horse to play with? Lame. It's the idea of the unicorn that's interesting, not the mechanical possibilities.
-Someone useful? Ok but then so either the unicorn is a unicorn--which is interesting, or is themself--which is interesting. But they're pretty much separate things that aren't better together. Unless it's a wereunicorn. Man, what a dumb idea. Next!
-Someone evil? Obviously this has merit. Then the question is whether the unicorn acts evil or just is a dumb animal. And, etiher way, there needs to be some threat of the evil person becoming a non-unicorn any minute.
The esoteric unicorn--it's like some sort of spectral 2D-seeming unicorn that sort of crosses your field of vision like it comes out of one side of a tree. Like a sort of heraldic image of a unicorn come to life. You could do a whole creepy sort of playing-card Camelot thing with weird stiff symbolic chivalrous types moving around and being inscrutable. Ok.
Victim Unicorn. You can be sure the bad guys are bad because they're fucking up a fucking unicorn. That's even worse than cutting down the magic pixie forest. The best bet here would appear to be to cruelly objectify the unicorn like in some Mackinnon/Dworkin wet dream (or would that be dry dream?) and just have the bad guys be fucking it all up and the beast itself has no agency at all.
In effect, the unicorn is, in this case, just a ground upon which the badguys can display their badness. Thinking up exotic badnesses to have them practice upon it is the hard part.
Unicorn-as-ingredient. The unicorn contains something of value, or something which will cause trouble if left uncollected. The unicorn can either be a victim (as above) in need of rescue or can be just out there and the PCs have to get it--in which case it gets to act like a regular monster (finally) and fight back.
Mutant unicorn. Like for example that creepy RuneQuest unicorn-head guy, or some sort of deformed abomination. Pretty much writes itself, since the creepiness and corruption is automatically embedded in the concept.
Important unicorns--like, unique ones which are exalted and meaningful to some group of people or citystate or reigion--are annoying. Ok, maybe not if it isn't obvious where they are. But having one just in the middle of town like "hey this is our unicorn", I hate that. Some random unicorn in a forest somewhere that you can't kill because it will make all the children of Grophyndoria grow a second head that will bite their first head, that's ok. Important but obscure is the point, I guess. Not a fucking mascot.
And if it glows or something, fuck it.
Trapper-Keeper Unicorn. So, it's like pink and sparkly and frolics. I sometimes think it'd be fun to have some acid-trip adventure where the girls have to fight some deluded princess in her horrible sparkly pink kingdom with kitten warriors on unicorns and clouds that cry rain on you. Not today, though.
Talking unicorns suck. What are they going to say? Anything a unicorn has to say is too funny. Can't do it.
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