Imagine horse riding as a skill in skyrim, with perks that add HP, speed, horse armor, hoof damage, etc?
I'd love that.

No thanks

An entire perk tree made for sitting atop of a horse...? No.
All horses do is run away and kamikaze a dragon...
A mini-perk like +10% mounted speed you earn by riding [distance], yes

Would make as much sense as a skill tree for marriage.

In it's current state horses are pretty unnecessary and skillpoints are few as they are.

Unless you need to scale a Cliff. Cos horses can climb the [censored] out of them bad boys!!

Yea, that and the fact that weight penalties doesn't apply when riding is the only reason I wrote "pretty unnecessary" instead of "completely useless"

that would a bit stupid we aint playing wow mate .

Muffled Feet = Horses are near silent.
Keep Stum = Horses no longer report your crimes.
Pack Mule = Horses add 100 to carry weight.
Billy Goat = Can climb a mountain (would have to remove it by default obviously)
Red Rum = Add 5% to sprint speed and stamina each level
Where's Wally = Can call a horse back to you twice a day
Guinea Pig = Can feed the horse ingredients to discover properties
War Horse = Can wear armour, bravery increases
Tap Dancer = 10% to attack damage each level
Mad Maximus = Can strap a machine gun to it, you know for immersion

Default setting changes = can't climb very well, loses health constantly when overloaded, runs away at the slightest thing.

Or just fix the horses so they don't do stupid [censored] to begin with.

i actually don't mind the idea, though horses would need a complete overhaul from the next to useless ones we have right now, horse combat would be a must too if a whole perk tree would be viable.

blutendesherz have some good perk suggestions. However I'd rather have shoes and upgrades for different purposes rather than everything being perk related. Like saddlebags that can either carry more items, or a twin saddlebag that lets your companion ride with you. And rather than "horse riding", call it "animal handling" so it can cover more.

Also, as a hunter in the game, I don't really feel like a hunter. Animals pop up everywhere and all I have to do is shoot them. They should be far more shy and player aware, and run away. So with a hunter skill tree, there could be special bonuses to animal stalking (sneak feels a bit too overpowered as it is once you take the perks), tracking (how visible the currently missing prints are, and for how long they remain visible) and so on.

I am all for more skills. They help define your character. Some call them useless - simply don't use them then. I Dungeon Master, we had a crapload of animal handling related skills, although each skill there (thousands) worked more like perks works in Skyrim.