Right-click on the Pokémon you wish to mount with no item in your hand. Left-shift to dismount (this is a vanilla mechanic). 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. Speed cannot be altered. Pixelmon will always travel in a straight line in whatever direction their head is aimed; for this a third person view is helpful. Like horses, your mounts have health. The amount of health they have is dependent on the amount of health the pokemon has. For example, a level 82 Jolteon will have more health than your level 1 Aerodactyl.
Riding a Shiny
Land mounts do not jump well, and move fairly slowly.
Water mounts typically function well on land, usually with smoother jumping abilities. Their water travel is not particularly quick. It is best to mount the Pokémon on land and then enter the water. Once you release the spacebar, your Pokémon will go back to sinking, and you will constantly need to hold it down to stay afloat. Gyarados is not recommended, as its model is very bulky and gets stuck easily. It cannot fly or jump easily. It does best in wide open water.
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 you get off the ground. There doesn't seem to be any stamina, so flight is unlimited. Highly recommended are Archeops and Pidgeot, as their heads and necks don't hinder first person view. Note: Pixelmon that hover (Aerodactyl, Flygon) sometimes require you to meet their level in order to mount.