Warning shot: Aim Weapon by holding RIGHT CLICK, then press U. Then LEFT CLICK for firing a warning shot.

Dodge on horseback: hold RIGHT CLICK to aim, then press SPACE to perform dodge movement on horse.

Horse rearing: hold left CTRL, then press SPACE bar.
Horse walk in place: Hold SPACE while stationary. Then hold LEFT CLICK.
Horse strafing: Hold SPACE + A/D
Calming horse: hold G all the time while in the vicinity of predators to calm the horse down, so it won't buck

slide stopping horse (requires higher horse bonding): hold SHIFT to run your horse, then press both SHIFT and left CTRL to stop your horse.

Backing up a wagon or horse: hold CTRL

Switch between hatchets (or also, change ammo type): select the hatchet, then press X

Pushing other players / npcs: first, run at another NPC by holding SHIFT. While running several times click LEFT MOUSE fast. You can have a weapon equipped while doing this, or not have any weapon equipped.

Shoving: be unarmed and press F while facing another player / npc.

Auto pilot: This is the most useful hidden command in the game. First press M to open the map and set a waypoint. Then ride your horse and put it in motion by holding SHIFT. While riding your horse or a coach, hold V for about 4 seconds. This will activate the cinematic camera mode, but also: now your horse will navigate on its own to the set destination on the map.

Blocking a blow in melee fight: hold R

Dodge-jumping: Hold RIGHT CLICK to aim, then press SPACE

Use the card "slow and steady" for sedating predators. The horse won't throw you off then.

Equip a melee weapon (like the tomahawk or the machete), then run your horse towards an enemy while holding F. Release F when you are side-by-side with the enemy to hit the enemy with your melee weapon from horse.