I would have guessed houserule, but PiXeL01 is correct, it's in RoCo II, page 18.
If you used that optional rule, it would be reasonable to also apply the usual difficulty modifiers (easy, hard, sheer folly...) based on the situation. It's easy to overcome the fear experienced seeing a single creature at a safe distance on a sunny day. It's harder when the creature is half in shadows, close by, and you don't know how many there are. It's probably easier when you have cast a spell to summon the creature which gives you some control, and harder when you summoned the wrong creature and realize it's beyond your power level to control what you got.