I keep seeing threads with "How about Horses?" "I want horses" "we should have horses" with no other information. You need to break you're idea down into what the horse does, how it would work and fit into the world as it is now.
So here's my idea, feel free to make suggestions to improve it or remove things to make it easier. But remember suggestions that are too complicated and may completely change how the game works...most likely would never be implemented, the main goal is to get something into the game then once it is in you begin suggesting improvements on the current model.
The Basics New Items
Lasso (Recipe: Crafted with 9 string)
Works on all animals, the lasso grabs up to 6 animals at a time forcing them to follow where you pull them. Can attach Lasso to any animal within 4 blocks of the player by right clicking. Drag length is 8 blocks.
Riders Saddle (Recipe: Basic Saddle we already have.)
Once equipped to the horse, the horse becomes a riding horse. Right clicking the horse will mount the player up and give the player control to move about freely. Dismounting will put the horse into an idle state, it will not move from the position it was left on. On SMP if a stranger hits your horse it will attack much like a wolf when it is hit.
Pack Saddle (Recipe: Saddle combined with 2 chests)
Once equipped to the horse, the horse becomes a pack horse. The horse will always be in a wandering idle mode, wandering a small area as unsaddled horses do. The only way to make a Pack Horse to follow is to lasso the horse and force it to follow. Right clicking the Pack Horse (without Lasso) will open its saddle bag to an inventory screen the size of a full chest. On SMP if a stranger hits the pack horse, it will run away keeping a distance of at least 5 blocks between it and its attacker at all times until a set amount of time passes.
Size: 3 Tall 1 Wide (will not enter tunnels unless they are 3 high
Colors: Various colors, with some rare colors you get with breeding
Speed: Steve Speed plus half
Riding Horse: See New Items Description.
Pack Horse: See New Items Description.
Spawns in the grasslands in herds of 3 to 6.
Can be tamed with apples, but like the Ocelot all the horses will run if you approach too fast. Once tamed the horse will follow you, right clicking the horse will make it idle wandering a small immediate area to the block it was idled on. Once saddled, the saddle cannot be removed and the horse will act accordingly to which saddle it is wearing.
Health meter will be judged by the position of its head and neck, fully up is a healthy horse, head down in a sad position is a horse near death.
optional addon for all animals
New grass (like tall grass) drops Hay Seed, only found in grasslands in clustered patches near water.
Grows on grass and dirt blocks, grows to two blocks high, when broken drops 2 hay items. When fed to animals it heals their hearts if they've been damaged. (Not used for breeding or taming, it is just an animal health item)
Made by combining 7 Hay and 2 String. Hurt animals will go to it to heal themselves, each hay bale has 5 full hearts worth of healing. It will go from bright yellow to brown as it is used up, once fully used up it will disappear.