Ok, so I've been to a few events and have continuously heard something about someone wanting their horse. Here are a few facts to consider...
Jastrey is small.. a horse needs 1/2 acre per horse in order to have enough grazing land to be healthy. A horse also needs grain and hay, even on that 1/2 acre. Performance horses (meaning war horses, which is what a knight would ride) need more. A typical horse eats 3-8lbs of grain twice a day in addition to grass grazing and 1-3 flakes of hay (approximately 10lbs or so). Feeding it would be very expensive as non-food products would have to be shipped over the verge and would likely become prohibitive. Due to all these facts, keeping a horse would be for the upper echelon of classes only, and there is likely no indigenous population as there would be intense inbreeding with such a small plot of land.
The verge.. horses have an innate sense of self-preservation. They will flee from danger in 99.95% of all cases. Some breeds flee faster than others. The war horses you read about from the middle ages needed intense training to leap into danger, and even those Medieval Times horses need that level of training to run straight at someone holding a sharp stick in their direction. People and playable races have been able to get across the verge but suffer so badly from its effects that they reach the brink of insanity every time they near it. Horses do not reason the way people do, and even the most well trained horse and one raised around it will still run away from fire as they know it's dangerous. Same as the verge. Getting a horse across it would take a LOT of magic and a lot more training. Keeping it from running away from one part of it just to fall off the other side of the shard would keep similar amounts of work.
Horses are not something you get on and instantly know how to ride. Like a bicycle, it takes a lot of work to get a horse to perform the way to expect. Lower level characters would likely never be able to have one due to the expense, difficulty moving and training needed.
In my eyes, horses do not exist on Jastrey for those reasons. This is personal and professional experience talking, not just observation.