Riding - Control Mount

Mounts, whether they are mundane, like horses, donkeys or camels, extoic beasts like griffons, hippogriffs, or giant lizards, are valuable assets for everyone who can afford them. They allow people to travel quickly between settlements, get an advantage on the battlefield, and drag or carry heavy weights.
Mounts, at least the mundane ones, are still simple animals that require domestication. Even then, pushing an animal to do something against its nature is not an easy task. When the animal's instincts take over, they won't be as receptive to their rider's commands.
There are two situations in which the rider has to make a check to keep control of their mounts. The first one is bringing the mount into battle. Riding the beast into a battlefield exposes it to the cacophony of violence and to a panoply of elements that a horse would simply flee from. Whenever your mount is exposed to combat, the character has to make a Riding check unless the mount was trained for combat situations.
Another situation is when the mount is facing an event that they find terrifying. Note that not all events are as frightening for an animal as they are for humans. Most humans would not run for their life during a thunderstorm. But a horse might panic.
A Control Mount check is made as an out-of-turn incidental when the mount is facing whatever makes them susceptible to panic.
The difficulty depends on what the mount is facing. The suggested difficulties are based on the behavior of a normal domesticated horse.
Note that war mounts are used to the chaos of the battlefield and their rider can generally keep them under control without making a check. Still, even a warhorse can start panicking if it is attacked.
The table below uses the term mount for riding beasts not trained for combat and war mount for those that are.
EXAMPLE OF A CONTROL MOUNT CHECK
Grayson is a travelling bard on a journey across The Kaldaria Highlands, stopping in each village, performing for the people. He is now on the road riding his horse. The day is cloudy and moderately windy. To spice up an otherwise uneventful journey, the GM spends a story point to have lightning strike suddenly, making the sky flash with light, followed by a downpour.
The sudden lightning startled the horse. The GM calls for an Average (dd) Riding check. This check has no special modifier.
The roll generates fa. The horse panics and flees in the opposite direction, which in this case is a forest and this lasts for two rounds, or two minutes. Grayson's player spends the a to have the opportunity to calm his mount before the end of the 2 minutes. Grayson doesn't know these woods and would like to avoid going too deep and ending up lost.
In order to calm his mount, he has to succeed on an Average (dd) Riding check. Since his mount is already panicked, he adds b to the pool. The GM spends a story point to upgrade the difficulty as these woods are treacherous to navigate. The roll generates shhd. The GM spends the d to have the horse throw Grayson off and make him land prone. Yet, the horse stops running and seems to regain its composure, thanks to the check succeeding. Otherwise, the horse would have continued its course, carrying away all of Grayson's belongings it had in the saddlebags. Finally, the GM spends the hh to have a trio of hungry wolves creep in from the underbrush, approaching Grayson with hungry snarls...