Some feedback: I not really like that high-tech late game mount: the pony. Its not that I dont like ponies, just imagine a frost giant riding this small mount, which will probably scale with the level of the f-giant into a giant pony. Also stiff-necked Serrane would throw away here proud warhorse for this circus animal? Nah!
Frost Giants (FG) can't ride ponies (nor, I think, Mire Skaths, Ravenous Harridan, and War Boar). They can still research it because there's a "wild mounts" improvement (Pony Stable) which they can build and because it's a prerequisite for the Hatchling Mounts tech. Given where this is the tech tree, I could remove the other arcane mounts techs for the FG tech tree and give it a unique Hatchling Mounts tech. However, this would give FG less improvements to gather the needed wild mounts used to build troops with hatchling mounts. I think it's not worth the effort given it will make wild mounts rarer for FG. If it's an immersion issue, just think of it this way, they build pony stables and trades the ponies for other wild mounts (in this case, hatchling mounts).
I will try to see if Faith Air / Faith Fire can mount ponies and block it if that's the case.
Beyond that and generally speaking, the arcane mounts techs use the "rare" mounts in the original FELH, mounts that you would only get in quests: Mire Skath, Pony, Ravenous Harridan, and War Boar. I'm guessing I added this before I created any mounts myself... There was obviously a humorous component in finding a pony in dark fantasy FELH and Stardock has been known for that sort of, hum, unexpected item (Bunny Slippers anyone?). I'm not a fan of said ponies myself, I used to never equip it on my main stack and would give it to some lone scout, but heh, feels like it's part of what makes the game at this point.
Apropos circus. I think its not fair that only those factions who has animal husbandry should be able to get the acrobat gloves, or in other words be able to cast faster. And I see not really a connection from acrobats to faster casting. That would rather be a kind of ledgerdemain. I would rename the gloves to something fitting and make it available in the sorcerer tower (for all) which simultanious would broaden the offer there.
Only factions w/ Undead faction trait don't have Animal Husbandry. In other words, only The Dead (or custom faction w/ said trait). That's about as limited a limitation as one can be. I also feel acrobats can be dexterous and dexterity can be useful when it comes to hands/casting? Maybe I don't know enough about acrobats, dexterity, and spellcasting however. Legerdemain is obviously be a perfectly appropriate word as well, but I was trying to create a "set" with the Acrobat's Pantaloons from Black Market Bazaar mod, so these won't change.
That said, I can show you how to unlock the gloves with the Sorcerer's Tower, that literally one line of XML to add.
Why not make the circus a place where all kind of mounts (for heroes) can be bought before they are even researched. (for a proud price).
Not convinced about this, but I will think about it. The main issue is it will either clutter the shop even more (and require me to duplicate all mounts) or it will need a bunch of spells to achieve it with the current mounts. Neither solution is very appealing as there is 25+ mounts in the game now.
I would also incease the local happyness a bit more to counter-balance the slum requirement a bit. Its entertainment after all.
That's not likely to happen, I feel the Circus is already quite good as is and the point of forcing you to take Slums is just that, to give you more unrest!
The pony could be a default mount for smaller races like gnomes or faith earth. (somewhat nerfed please), while the f-giants really shouldnt be allowed to ride anything but their beast type mount and maybe some other monster type-mounts. Anything else would look silly and unimmersive.
As said above, don't think FG can ride any of the arcane mounts. But I like your idea of a "pony" mount for Faith Earth.
Would probably add a Mountain Pony in a future update.