Don't even bother to find them. Trust me you'll waste time. Whenever you see them just hurry up and call ur argentavis(or any dino that can carry the unicorn) then carry them to a safe place or your base and tame them. If you see them on a dangerous spot and you're still requesting an argent(or any dino that can carry the unicorn) then you have to choose 3 things. protect it with everything you've got, kill then take its implant, or go out of the area until you can't see the unicorn so the area you left will freeze (not so sure about the 3rd option but it's a theory, and i'm so sorry if it didn't work.)
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.