Right-clicking on a mountable Pokémon with no item in hand will cause the player to ride the Pokémon. Right-clicking again will dismount. Occasionally, this will not work, in which case it is best simply to recall the Pokémon and send it out again. A saddle will not work and is not needed. Turning must be done with directional keys and can be somewhat difficult. Pokémon will always travel in a straight line in whatever direction their head is aimed; for this a third person view is helpful.
Land mounts do not jump well.
Water mounts typically function well on land, usually with smoother jumping abilities. It is best to mount the Pokémon on land and then enter the water. To stay afloat, the space bar must be held, or else the Pokémon will start to sink.
Flying is similar to Creative mode - the jump button must be held in order to ascend. It can be tricky to control. Flying mounts are by far the most useful. The movement is very quick and smooth once off the ground. There is no stamina, so flight is unlimited. Highly recommended are Archeops and Pidgeot, as their heads and necks don't hinder first-person view. Note: Pokémon that hover (Aerodactyl, Flygon) sometimes require the player to meet their level in order to mount them.