This community stands for me to ban you, and anyone else whom I disagree with, part of my powers to abuse really.
Seriously though, this place exists for discussions and for people to hash out ideas, though in the past many people tried to get people to do things for them so regrettably it is commonly an assumption made that would be the intent. The reality if one wishes to have a discussion about an idea but doesnt wish to get someone to make it, that's fine by me just the same.
The problem with this horsey idea is namely in if you wish it to look crude or pretty. In short, yes I can force hoop at gunpoint to fudge the concept into a crude but serviceable horsie with a cop on top but in my eyes that's missing the intention a good bit.
To do this proper like would require far more work than that, in reality to make a really nice addition it would require a few things:
The horse it's self might be usable from the stock factory horse in the game, that's debatable as I've never looked at the horse it's self to see what it's got going on. For the sake of argument here we'll assume she's usable though.
Next it'd need a saddle added, all in all not that difficult assuming the horse it's self is usable and doesnt "bound" or gallop when it moves, if it does that would be a serious hindering problem to overcome, ultimately requiring the saddle to be mounted onto the horse, which may not work so hot if she does gallop, rear or anything of the sort.
Next up would be a rider, I've made one before for the motorcycle found in LA or NY mods (same guy/same bike in both mods just modded for ny)... I had to make that guy from scratch for the bike, he was a new model, not an existing em4 model because of what was needed to make him mount since he's animated to do so.. Let's say that you wanted to do this the easy way and plop him on top the horse in saddle, no mounting.. It still requires making a person model that fits the bill and designing him in such a way to fit on the horse (without the horse being present unless you have Zmod3 which most of the modders don't).
Worst case for the rider you'd want him to mount the thing, which would require someone with a whole lotta patience to move the guy in a relatively fluid matter, this game you have to animate using pretty crude techniques for most modders around, so it's seriously painful to do it.
Now, worst case scenario here:
The horse is bad, and won't be possible to use at all... He just is a bad horsey and is lame so we put em' down... *throws flower on po' horsey's grave..
This is the part that really sucks here.... If the horse is junk as far as the concept goes and we'd need a new horsey from the ground up... As a modeler myself I'd want to shoot myself to be blunt, but it would be possible to make the horse with relatively little pain.. The fun part would be that I would then have to animate his actions/movements to be as fluid as possible. That's alotta work, but the good news is the saddle nor the person matching is not an issue one bit compared to the horse.
Then the scripting it's self, for the easy route is relatively easy to do in of it's self because in essence the horse would become a person (means set speed and all that jazz though) If you made it a vehicle it would be far more intensive to sort out.
The harder route is a ton of work because you'd basically have to script the horse's movements for every single movement he does, along with triggering the animations for his movement and all that stuff, a serious task to undertake but ultimately the end result would be far far better..
My final thoughts... To be totally blunt for me and my perspective this task to do it proper and have a really high-quality end result is a ton of work for something that in essence is purely cosmetic... Granted, it could have some cool and fun features for crowd control and things like that, but for the work involved I wouldnt undertake this task.. This task would take someone with moderate to advanced skills for both modeling and scripting to do right, and a good bit of time for quality-control and testing to make sure it did not bug out. If you wish to try to undertake the task, I wish you the best of luck as it would be a cool cosmetic addition but a difficult one to do right.