Profession mounts are created by characters who are very skilled in certain professions. (Technically, the Sea Turtle isn't so much created as fished up, but the concept is the same.)
Most of these mounts can only be used by their creator. The Mechano-Hog and the Mekgineer's Chopper are the exception: they are created by engineers but can be used by anyone.
Click on a mount to learn more about it.