And isn't Kislev proud of their horses in particular? It's taking something they are proud of doing well and saying that this magic item does it better. Imagine some foreigner offering the Karaz Ankor some sort of magical fabricator, or offering the Cult of Taal a magical GPS for navigating forests, or offering Stirland a magical beer warmer.
In general, Kislev and Bretonnia have the most exceptional horses in the Old World. Bretonnians are excellent for having been crossbred with Elven Steeds long ago, and they are exceptionally muscular. They're the only horses who don't slow down at all despite being fully barded (no movement penalties from barding in the tabletop). At least for their chargers. It makes their Knightly Charges much more devastating because their horses are ripped as hell.
Kislev is the opposite. They have tough horses with great endurance or horses that are really, really fast depending on what their purpose is. Ungol Horse Archers want speed so they can strafe the enemy and kite them. Winged Lancers want endurance so they can keep charging. Both horses are exceptional, and it's demonstrated by their horses being faster and their cavalry stronger than the Empire's in Total Warhammer.
Another fun fact, Ostermark and Averland are the premier horse breeders in the Empire. Ostermark breed horses in the Veldt, and their traditions there cross over heavily with Kislev because the border used to be pretty flexible between the nations. Ostermarker horses are quite excellent and share a lot of similarities with Kislevite horse breeds.
Averlander horses are bred in the southern plains of Averland. Less is known about them, but the Brigundians, the pre-imperial ancestors of the Averlanders, were famous for their chariots, so good horses were a given.
I would say something about Arabyan horses here if I had anything to say aside from vaguely remembering something about them. I know there's a special Pegasus called the
Radiant Pegasi who absorb sunlight and redirect it, and I imagine they probably have good horses, but you know. Araby.