On 10 Summer, Freyr's Horse had a single heart and was painfully slow to ride. On 11Summer, when he saved his game, he was astounded to discover that his Horse had the maximum Friendship Level of 5 Hearts and could run like the wind. He had not made any Wish with the Goddess with respect to the Horse, but had brushed it, whistled to it and mounted it.
11 Winter was a rainy day. All my animals were inside the Barn and the Chickens were inside the Coop. While caring for the Livestock, I accidentally pushed the Horse out of the Barn. I used the Bell instantly to bring him back into the Barn, but apparently the damage was done in terms of his FP.
Although, when I 'talked' to him, he was in a good mood, on the morning of 12 Winter, when I saved my game, my Horse, who had 5 Hearts on the morning of 11 Winter had only 1 Heart on 12 Winter.
In other words, the FP 'Bonus' can work both ways. The accidental effect of pushing the Horse out of the barn, however briefly, was multiplied to the point where he lost 4 out of 5 of his Hearts!
I reloaded and replayed the day, mainly because the Horse's speed is an integral part of the game, especially when you need to race against the Robot Horse.