If it won’t allow you to feed it, knock it out and let it wake up then try again. Most likely due to the fact someone else tried to tame it before. Also, it tells you when to feed it at the bottom corner of your screen.
I encountered one on Ragnarok, freaked out tamed it and was happily riding it back to my base when a stupid Raptor decided "I'm about to ruin this man's career" and killed my Unicorn. I have yet to find another one and I have a gravestone for my lost Charlie.
Just showing Elizabeth that anyone can post here so this comment section is NOT official information, but player submitted tips that can easily be wrong if not fact checked. Unicorns have been addressed by Wildcard and do NOTHING for twins and triplets.
Not even 10 minutes in, one of these things was galloping on the other side of a lake, I couldn't see it from close, but it was clear it had a horn, then it got chased off in the forest by a dilo. Goodbye, dream of a unicornthatIwouldn'tbothertotameanyway.
Who thinks this should have a charge attack that can aim for vitals and can temporary cripple creatures and should have a unicorn dust ability that befrends very small - medium creatures to help you fight and to give yourself a point to tame this rare creature