First you have to be in Apaloosa Plains to be able to see any unicorns, then you have to get this secret trait called like wildlife lover or something, by being friendly to all animals you see. It wont show that you have that trait, but when u spend like a whole day interacting with pets, at about 10:00 at night, go to the fishing areas like crystal springs or something and then wait for the cloud to appear. If it doesnt, try again the next night, because unicorns only have a 40% chance to appear each night.
Hope this helped!