I have a large enclosure with no roof and tall walls. I used Argentavis to pick up the Unicorn. Dropped it into the enclosure then went in and gave it a carrot (while wearing Ghillie armor). That’s the best way to tame it. If it gets spooked, just leave the enclosure and come back later. It can’t run away and you can try as many times as you need.
As a mobile player I feel the pain of never being able to tame this do to it needing griffin kibble we the ARK mobile players DEMAND a change we want to tame them with carrots like play station players can arrow if you support
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.