PHB page 198, mounted combat:<br/><br/><strong>Mounted Combat</strong><br/>A knight charging into battle on a warhorse, a wizard<br/>casting spells from the back o f a griffon, or a cleric<br/>soaring through the sky on a pegasus all enjoy the<br/>benefits o f speed and mobility that a mount can provide.<br/>A w illing creature that is at least one size larger than<br/>you and that has an appropriate anatomy can serve as a<br/>mount, using the following rules.<br/><br/><strong>Mounting and Dismounting</strong><br/>Once during your move, you can mount a creature that<br/>is within 5 feet o f you or dismount. Doing so costs an<br/>amount o f movement equal to half your speed. For<br/>example, if your speed is 30 feet, you must spend 15<br/>feet of movement to mount a horse. Therefore, you can’t<br/>mount it if you don’t have 15 feet o f movement left or<br/>if your speed is 0.<br/>If an effect moves your mount against its w ill while<br/>you’re on it, you must succeed on a DC 10 Dexterity<br/>saving throw or fall off the mount, landing prone in a<br/>space within 5 feet o f it. If you’re knocked prone while<br/>mounted, you must make the same saving throw.<br/>If your mount is knocked prone, you can use your<br/>reaction to dismount it as it falls and land on your feet.<br/>Otherwise, you are dismounted and fall prone in a<br/>space within 5 feet it.<br/><br/><strong>Controlling a Mount</strong><br/>While you're mounted, you have two options. You<br/>can either control the mount or allow it to act<br/>independently. Intelligent creatures, such as dragons,<br/>act independently.<br/>You can control a mount only if it has been trained<br/>to accept a rider. Domesticated horses, donkeys, and<br/>similar creatures are assumed to have such training.<br/>The initiative o f a controlled mount changes to match<br/>yours when you mount it. It moves as you direct it, and<br/>it has only three action options: Dash, Disengage, and<br/>Dodge. A controlled mount can move and act even on<br/>the turn that you mount it.<br/>An independent mount retains its place in the<br/>initiative order. Bearing a rider puts no restrictions on<br/>the actions the mount can take, and it moves and acts<br/>as it w ishes. It might flee from combat, rush to attack<br/>and devour a badly injured foe, or otherwise act against<br/>your wishes.<br/>In either case, if the mount provokes an opportunity<br/>attack while you’re on it, the attacker can target you<br/>or the mount.