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Finally found that the Stables of Vigor adds 'skills' to any mounts built at this city. They get tagged with Vigorous Mounts and the skills are Free Movement (no terrain penalties), Fast Healing (+6/turn) and High Morale (+200). So don't waste time building until you are ready for those units. It might be worth it to create an Army of these for fast response unit but otherwise, while in a normal mixed army, only the fast healing is a plus for me.
 
Now that you've experienced by yourself, please know that there's an in-game compedium called Tome of Wonders (ToW in short). You call it from the third icon in the right upper corner, or by (right-) clicking on objects. Stables of Vigor are under the Locations tab. You'll find also usually more detailed information on the Wikia.
Have a good game !