TL;DR: Not possible with a plugin, or even a mod, probably.
I love horses. Anyone who knows me knows that I love horses beyond anything else in the world.
That being said, horses in video games suck. Always.
There are multiple advantages to having a horse in combat in real life.
1: Speed. If you charge someone on a horse, and hit them with your weapon, it does a LOT more damage to them then hitting them with a forward step like you would on the ground. As well, you can chase anyone down who tries to run, easily. With a bow, it will make you very hard to hit, and you can pick off your enemies safely and easily.
2: Intimidation. Have you ever seen a horse in real life? They're huge, and terrifying. Now imagine one running toward you, with an armored guy with a weapon planning to kill you on top. Formations often crumbled, and people routed, just from being faced with a cavalry charge.
Yes, that's it, that's all they needed to become some of the msot powerful weapons on a battlefield, but as such, there is downsides as well.
1. They are very poor with water. Horses are plains creatures, they do not move fast in water, and water can stop cavalry. If you have a river between you and the enemy cavalry, and there's no bridge, they're probably not going to try to cross. But, with that, it's hard to get any army to cross a river in general, horses are just worse.
2. They're difficult to take care of. Having an army of cavalry is far more expensive than having an army of infantry. You need provisions and people just to take care of the horses, and they're big and take a lot of food. If you don't have supplies, you can't have a lot of horses.
3. You stop, you die. If you stop on your horse, you lose all your advantages. They're not fast when stopped, and they're not nearly as scary. And with that, they're very easy to kill. Most horses have to be lightly armored or unarmoured, or they can't move very quickly. Cavalry are only good in spread out formations. If you charge your horse right into a group of soldiers, you may kill a few, but you die too. There were heavy cavalry, but they're even more difficult to take care of, and make your army a lot slower, while only helping out with the intimidation factor, really.
Now that being said; How fast you move in minecraft has nothing to do with the damage you deal, and you're more mobile with an enderpearl than you are with a horse, no matter what changes you make, making chasing down routers impossible, because they can move where you can't. Now, knowing that, are you going to be scared of a horse? No, probably not. This is minecraft. Horses aren't huge, and you won't actually die. They won't have any more advantage, either.
Still though, they are poor with water, and forests (which they're not the best with, but can easily manage in real life due to roads). And again, in minecraft, if you stop you die. That applies on foot too. But in minecraft you have to move side to side, which is pretty hard with a horse in the game.
Minecraft isn't the only one plagued with these problems. Skyrim as well (although horses are the fastest way to get around still so they have some use) has terrible horse combat, although that is mostly due to inability to hit your enemy while on the horse, as well as the lack of speed bonus.
In fact, the only game I have ever seen that does horses well, is mount and blade. Horses in mount and blade are a formidible foe. There is speed damage, and if you have a lance, you get enough speed damage bonus to almost insta kill anyone, if you're going fast enough, making them intimidating again, especially since the size is better as well in that. And, they have their downsides too. Horses move very slow in the shallow waters of the game, very, very slow. It will take away that speed bonus entirely. They're also more expensive in terms of money. And, spears stop horses. You get stabbed with a spear, your horse stops. And once your horse stops, and you can't get away, you die to the infantry around you.
So, unless minecraft almost completely changes their combat engines to a more realistic, mount and blade style combat system, horses will be very useless in it. This can't be done with a plugin.
Also, no, buffing horses will do nothing.
I have buffed horses, I have 2 ironhoofs, and horses bred from Ironhoofs. Hundreds of health, very formidable, and very fast. Still useless.
Although, I do like the idea of buffing horses by feeding them, if the effect is permanent. Might make taking care of horses and farming them and such neat.
