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.
Person who asked where to find unicorns and wanted one for their equus farm. The unicorn can appear anywhere a normal horse can as well as on cragg's island, herbi island, and the dead island(also known as carno island carnivore island and rex island) only one unicorn can spawn at a time but if you tame one it spawns a new one somewhere and you can breed them! Happy taming 🦄