A horse is a domesticated animal, often used as a mount or pack beast. In the novels, it is the primary method of transportation. Distances are also often measured in multiples of a "day's ride", which is said to be between 35 and 50 miles per day for very good specimens and perhaps closer to 20 miles per day for "average" animals.
In The Witcher 3: Wild Hunt, Geralt can use Axii on wild horses to then ride them. Interestingly enough, this will cause the horse to be permanently ridable assuming it doesn't de-spawn. It can also be used on multiple horses, so Geralt could essentially "tame" an entire herd.
↑ 1.01.1It's a common misconception to call a horse with a white coat "white". In actuality, a true white horse is very rare. One noticeable difference is that a white horse has pink skin while a gray horse will have black skin, even if their hair coat may be white.