the AI could use them (I think), it just need a predefined sets of units that will follow the rules  of creation

My thinking is that changes should be kept as simple as possible. First of all, since the game treats all horses as warmounts , I would run with that accordingly.

All mounts (horses and wargs) should have an additional 20-30 labor cost for units and a 2 Gildar/turn wages cost (reflecting expenses like fodder, grooming etc.)

I would assume that all Soverigns and Champions would have the skill to ride horses and fight mounted for game purposes.

As for as roles go, I suggest the following:

Wargs act basically as a fast mounted close combat unit; they rush in close and maul enemy units, striking quickly. Wargs however are not as strong as horses so they have less HP than horses; however they incur no penalty for dodging since they are built lower and are more aggressive.

Horses would have either two roles: as heavy cavalry, designed for shock/charge attacks or as light cavalry; horse archers/mages. Heavy Cavalry would have heavy armor and weapons with barding for the horses, also with bonuses like charge etc. Light Cavalry would have light armor, weapons like bows, staffs etc. and generally act as skirmishers, shooting at enemy units from a distance and avoiding close combat. Horses should give more HP than Wargs but incur a -10 penalty to dodge (bigger target).

What do you think?

I think that it doesn't have to get too complicated mechanically to add some variety and depth to the late game "cavalry charge" but some changes would really improve this part of the game and make you think a bit more on your army composition.

Increasing production costs or maintenance makes sense when thinking about the care of the mount and the added equipment that would go along with using a mount,but I don't think that alone would be enough to deter people from using them the way they do now.

You could also reduce the max size of the unit by 1 if it is mounted to reflect the ability of infantry to work in tighter and more numerous formations.

It makes sense for spears to have a bonus vs mounted units, but right now it seems that the most commonly used melee weapons are swords/daggers followed by spears and then axes and hammers really not being used as much.  I think if spears were buffed like this then people would only really choose between swords and spears.  I would still be in favor of adding in a mounted bonus to spears as long as the other weapons were looked at also to rebalance.

Mounted combat could also be by expanded upon by adding in other kinds of mounts like a dragon mount, giant turtles, or special horses/wargs in a limited fashion through quests, random events, rare resources, unlocked by techs, or spells to summon.

I don't create swords too much either for trained troops unless I have the unique tech for it, but I do use them a lot on champions as I find a lot of good daggers/swords for them in the world, especially for the casters.  I find them effective when you can buff their attack through spells or items.  For trained troops I usually use spears with mounts and charge.  Otherwise I do think the value of having a high attack is generally undervalued, but spears would still would be too good if they gave a bonus to cavalry in addition to armor piercing and no counter.

Maybe create a seperate series of lances that only works for mounted units?

It bugs me that the warg is tech-wise more advanced than the horse, but in the lore of both this game and general fantasy, wargs are used by the more primitive tribes.

So let's try to separate the tech tree like so:
Husbandry: Enables both horses and wargs. Prerequisite for Mounted Combat.
Mounted Combat: Unlocks the Stirrups equipment and Mounted Combat trait.
War Breeding: Enables use of the War Horse. Dead end tech.
Battle Training: Enables use of the Battle Warg. Dead end tech, cheaper than War Breeding.

The animals themselves would have the following penalties and costs:
Any mounted unit: -10 accuracy, 50% pierce vulnerability.
Horse: 2 g/s wage, 10 additional production.
War Horse: 5 g/s wage, 25 additional production.
Warg: 1 g/s wage, 6 additional production. 
Battle Warg: 3 g/s wage, 18 additional production.

The strategic benefits of horses (faster strategic movement)  will mostly outweigh the difference in cost of both animal and technology, and it serves to keep the two separate, both fluff-wise and mechanically

The equipment and perks should be like so:
Stirrups: Removes the accuracy penalty. 4 metal, 4 production.
Mounted Combat: Removes the vulnerability penalty. 6 production.

I like The_Regicide's idea too.

1.Yes, I agree, that horses should have their own upkeep. For example 1 gold.

2. Mount should not give opportunity to carry additional weight. Instead every mount should give weight and accuracy penalties to its rider. Riders need to be lighter in weight to ride horses.

3. Riders should not be able to use large shields. Perhaps every type of shield should give less bonuses to mounted troops.

4. Riders should nor be immune to bash strike which makes them prone. Otherwise maces are less useful.

Infantry should have opportunity to have better armor and be more accurate.