Intelligent creatures, such as dragons, act independently.
You can either control the mount or allow it to act independently. While you’re mounted, you have two options. Otherwise, you are dismounted and fall prone in a space within 5 feet it. If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. If you’re knocked prone while mounted, you must make the same saving throw. If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. Therefore, you can’t mount it if you don’t have 15 feet of movement left or if your speed is 0. For example, if your speed is 30 feet, you must spend 15 feet of movement to mount a horse. Doing so costs an amount of movement equal to half your speed. Once during your move, you can mount a creature that is within 5 feet of you or dismount.
