I kind of like the idea of simply having alternate art for the arcane line and the Horselord promotion to grant extra speed and a token amount of withdrawal. Here you go, they look like they're on horses, they're faster with a bit of withdrawal, but they won't be as good as the true Mounted line for jobs the Mounted line ought to be doing (raiding), and they can "mostly" keep up with the army assuming they buy a Mobility promotion and aren't trying to tag along with Mounted units who themselves also have Mobility.
On this basis, they might not need any extra hammer cost, level prereqs for upgrades, anything funky about unlocking Rancher building promotions, etc. Again, you're a Hippus arcanist of some sort, here's your alt art and a promo that the core of your army has for free as well anyhow. Quick and dirty, worst case it ends up a testbed for further expansion (i.e., of being able to buy all the withdrawal boosting promos, or Rancher type equipment, or being "nightmare" mounted, etc.)