The ability to use a mount is essential to participation in cavalry units, and to swift travel through the Empire. It is a utilitarian skill, taught in most bushi schools in the Empire without having any real degree of prestige attached (save among the ranks of the Unicorn Clan, who consider an inability to ride roughly equivalent to a crippling physical disability). In general, all samurai can ride a horse at normal speed on level ground without the need for a Skill roll - the Skill only comes into play when riding a horse in difficult or stressful situations (bat terrain, thunderstorm, combat, etc.).
Individual types of horses react differently to various circumstances, and some can be much harder to control under stressful situations such as combat.