I agree with Riddles. Like any animal that has been living in desert environments for thousands of years, they've acclimated to survive in such. Thus, those that are still in these environments can probably go without water for longer periods than say, a Quarter Horse that lives in California. However, I think that if you were to take a domesticated Akhal-Teke (domesticated in this sense meaning always having water readily available) that lived with our Californian Quarter Horse, I don't think you could just take water away from our Akhal-Teke and expect him to survive without any issuses. In order for an animal to maintain its original instincts and abilities, it has to use them. Like with humans, you use it, or you lose it.