Legacy of Serrane Gifts Warhorse. Unless Serrane pulled her caravans with Warhorses, and Captiar built their roads on the backs of WAR horses, it would be more fitting if it was a heavily bodied, strong work horse. Serrane's caravan immunity would make more sense if their horses were strong and capable of out running attackers, as opposed to being mean looking battle equus.
Integrating a Stable into a city might cause the local citizens to start using them as a laboring tool. As apposed to randomly deciding. "You know, I didn't want to walk all the way over to that tile to train warhorses that end up getting brought here when we train soldiers anyway!" Local availability and surplus to be exploited for labor/gain isn't a 'fantasy element' however.
Regular Horse pretty much is a warhorse. Since they are 100% capable of fighting in an combat situation.
Unless of-course (the nerf:) that the regular type of horse was dismounted upon entering combat. Making horses mostly useful for traveling the world map and preventing non-mounted units becoming obsolete if you manage to acquire horses. Not to mention being pure gravy with 0 downsides.
This seems to be a pretty heavy nerf. If both changes are made, the horse tech could allow a 3rd horse which is THE warhorse, requires a horse resource and some production/training time, and adds a little unit upkeep to represent its power in combat. Uugh, the bonus of having the workhorse is that it can function as the warhorse only with the carry weight bonus instead, without spending extra on warhorse (since they'd be part of the city workforce anyway).
Obviously the nerf could (and maybe to prevent imbalance, even though it requires more tech) apply to the Warg.
nb: The Workhorse thing is more of a nitpick. Unimportant details. So hard to handle. Feel free to ignore it. 
