I wanted to open discussion on this topic. I feel that mounted units are in need of a bit of rebalancing.

Given the option, why would you ever choose to not have a mount ?

Depending on exactly which mount you have, you get extra movement, immunity to being knocked prone, and (depending on exactly which mount) bonuses to evasion, initiative, and/or carry weight.

As far as i can tell, there isn't any downside to these things. Mounts have no labour cost, they're a resource. As long as you have sufficient wargs or horses, putting your troops on them costs nothing.

This may seem realistic at first glance, but you have to consider the training too. The horse ranches we have, are those training the horses, or simply breeding them and providing them raw?  And why does having 10,000 unused horses sitting around, not cost your empire something to maintain them

I'll be doing some research to check if there are any hidden penalties i'm not aware of (or if any of the following ideas are already in the game!), but as far as i'm aware right now horses and wargs provide pure bonuses and no penalties.

It's generally common knowledge that mounted units are an inherently offensive thing. You don't put horsemen on your battlements during a siege, they charge out and attack the enemy.  To that end, it seems a bit odd that they don't have any defensive penalties. 

I'd propose (taking a leaf from Civilisation IV) that horsemen be stripped of certain defensive bonuses. For example, they shouldn't be able to "defend" (usually accomplished by not taking an action) and thus get no extra armour for not taking an action. They could also not be allowed to counterattack, which would give them a reason not to use the ubiquitous sword, and farther strip them of a defensive capability

In addition, spears. Fighting someone who's much higher up than you is difficult, against most weapons he would have an advantage. But long weapons such as spears and pikes, have the reach to negate this. To that end, i think spears, and similar long weapons, should get bonuses to accuracy and damage, when striking mounted units

Lastly, there is maintenance. This one is pretty logical. a horse is about 3x the size of a man, and it eats considerably more. so it only makes sense that bringing a lumbering beast with you everywhere you go should incur some extra cost. to feed it,  keep it with fresh horshoes, etc. I propose that each horse in active service add some amount of gold maintenance per turn. Say... 0.2 gold per turn each. This would effectively double the maintenance cost of the average mounted unit, versus the same group without a horse.

These are some ideas to offset the bonuses of having a mount. I'm not saying implement ALL of these penalties, but perhaps at least some of them would both make sense and help to balance things out.

That aside, the second thing i wanted to discuss is: Where are the extra mounts? In the game, in certain situations, you can meet champions which are riding Skaths, Harridans, and even Dragons. But i've not seen any ability for the player to use any of these things as a mount. The resoln empire can train lots of spiders, but they can't ride them as far as i'm aware