Sky Turtle

Sky turtles are a group of related species of flying terapins. they range in size from the size of a grown man's palm to approximately a mile across. Most of them are herbivores, munching trees, and sifting algae from the oceans. Only one species is known to eat meat on purpose. 
If a nest can be found before hatchlings emerge, it is possible to capture and train one to eventually be used as a flying ship, but this takes the better part of 20 years, so is rarely done.