When mounted, clicking on a fence will leash the horse to it. Requires Lead in the special slot that's added to horse inventory.
When the leash is removed or broken - Lead will not drop and will remain in the slot.
1.21+
Copper Horn is back. And it can be used to summon a horse from any location in the world.
Note: you bind the horse to the sound, not to the horn itself. If you lose the horn, you can call the horse with new one, provided it has the same instrument.
1.21.10+
No longer has instruments - horn has single unique sound instead. Binds only one horse.
Up to 1.21.11
Ridden horses can swim now. Press jump (spacebar) when in water.
Camels, Skeleton and Zombie horses are exceptions, by default. Configurable with '#horseman:cannot_swim' entity tag.
Horses now fit in boats.
1.21+
Makes traversing forests easier by tweaking Leaves collision. You'll get stuck on leaves much less now. Or you can turn off all leaves collision when mounted, if you hate them that much.
Increases horse step height by 10%. Allows riding up a path to a full block and not get stuck.
When mounted, your mining speed will remain the same as on ground. Removes 80% block break speed penalty when mounted.
Keeps player's looking direction when mounting a horse. Rotates horse instead of you.
When horse is not moving, player can move the camera independently of the ridden horse, up to 100 degrees (after which horse will follow the rotation).
Reduces slowdown when riding down blocks (looks like opposite of step-up).
Keeps greater portion of horse's ground speed when airborne. Improves horizontal jump distance.
Fixes jitter and reset back when riding up blocks (especially stairs). Mojang bug: MC-100830
Saddled horse will not go too far from where it was left by it's owner. Limits horse wandering distance.
Default radius is 16, to not make hitching obsolete.
Makes horse disappear (almost) when rider is looking down. Watch your step!
Adds a button and a hotkey to switch between horse inventory and yours.
Improves GUI when mounted: Jumping meter will only show up when jumping; Makes hunger bar always visible;
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Slightly improves visibility by lowering horse head position as well as rotating it forward.
Using Shears on a Donkey, Mule or Llama with a chest attached will remove the chest and drop it's contents.
1.21+
Tooltip with horse stats will show when holding a food item that horse likes.
At least they a considered as such in Minecraft's code.
That means that most of the features of Horseman are applicable to these creatures as well.
Author of Horse Buff is stopping development on it, and asked me if I want to include their features in my mod.