As this is my first post, allow me to congratulate you on an overwhelmingly fun and bug-free beta release! Having said that, as I notice issues I shall definitely report them.
I created a custom faction with the "Legacy of Serrane" trait. This enables bazaars (which work perfectly fine) and warhorses. I am quite able to equip all of my units with warhorses. However, the Champions and Sovereign are unable to purchase them! I tested the save by researching Warg Riding, and the Champions/Sovereign are able to equip Wargs without any issue.
Horses do not even show up as options under the Mounted Warfare technology because I had Warhorses available to my units starting at the very beginning of the game. It is my understanding that Warhorses replace regular horses.