Bored chemist, much as I hate to do this, you are in error. I have it on good authority that there is only one unicorn. Hence the correct expression is "the unicorn did it". I don't have to remind you of the rigour needed in all such scientific matters, correct units, correct dimensions and of course correct unicorn.Please state your authority.
I have it on excellent authority, my granddaughter, that multiple unicorns exist, apparently in many colours. However, I will concede that it is possible that only one unicorn was involved in the action (usually the pink one if I recall @Bored chemist correctly), so your expression would remain correct.
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