A knight charging into battle on a warhorse, a wizard casting spells from the back of a griffon, or a cleric soaring through the sky on a pegasus all enjoy the benefits of speed and mobility that a mount can provide.
A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules.
Once during your move, you can mount a creature that is within 5 feet of you or dismount. Doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to mount a horse. Therefore, you can’t mount it if you don’t have 15 feet of movement left or if your speed is 0.
While you are mounted on a creature:
While you are being ridden by another creature:
While you’re mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently.
You can control a mount only if it has been trained to accept a rider. Domesticated horses, donkeys, and similar creatures are assumed to have such training. The initiative of a controlled mount changes to match yours when you mount it, and it moves as you direct it. You use its Speed instead of your own, and you and your mount share movement on your turn. Remember that mounting or dismounting costs half your Speed when choosing to move after you’ve jumped onto or off the creature.
A mounted creature under your control can take the Dash, Disengage, or Dodge action during your movement on your turn. Additionally, you can use a bonus action to command the creature to make an ability check, use one of its special abilities, or take the Attack action (this does not include Multiattack).
An independent mount, meanwhile, retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. It might flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes.
In either case, if the mount provokes an opportunity attack while you’re on it, the attacker can choose to target you or the mount.