There are some myths regarding the random items you can collect. Perhaps there are certain things you can do to increase your odds, but after so many hours of playing the game (and wasting two hours on Rainbow Slugs on one playthrough...while getting four of them without effort in another) I'm inclined to think things like location/hour/weather are mere superstitions.
And it was stupid of the developers to make certain items into unicorns.
My only advice Mothman is to keep trying. Not very helpful, I know.