Curiously, the societies that used horses as a primary transit mode beat out the ones for whom oxen were the primary transit mode. Horses in Europe in the early middle ages were so specific to the noble class that it was assumed a guy with a horse was noble, and a noble without a horse was just a commoner. (This was before the merchant class messed the hierarchy up.)

So yeah, when the Mongols came with their society in which everyone was on horseback, it was a game changer.

Curiously, with the rise of firearms (involving centuries of tech races between crossbows and firearms vs. heavier armor) there was an overabundance for a while of heavy warhorses. Then someone invented the shoulder harness which allowed horses to pull a load without getting strangled.

That said, it's always neat to see when fiction writers actually do their worldbuilding homework and work out details like personal transit, freight, mail and so on and how they are affected by the diegetic mythical aspects.