Dragons

Some of the biggest—and most iconic—flying animals in all of history. And non-history. From
How Dragons Fly: When Biology Trumps Physics - Naturalist

House of the Dragon got me wondering about whether you could actually have a dragon. What would it take? I'm going to assume they are carnivorous being as they've got a reputation for eating people and large animals. You also get more energy from eating meat than from eating grass, and if you are going to be flying you are going to need a lot of energy. Like other large predators, they probably spend a lot of time lazing around and sleeping and only go hunting when they are hungry. Given their large size, they probably wouldn't have any difficulty finding something to eat, so they would not need to be airborne for long. Pop up in the air, take a quick look around, spy a fat cow, swoop down and chomp it up, pop back up into the air and head back to the cave for a week long nap.

If you have a tame dragon, as long as you gave it a cow every week or so, it would probably be very happy just hanging out in its cave, dreaming ferocious dragon dreams. You need it to go on a mission, you give it a couple of pounds of amphetamines and a barrel of sugar water or alcohol and he'd probably  be good for an hour. If you can't complete your mission in an hour with a dragon you probably need to rethink your strategy. You can probably only ask them to fly a mission maybe once a week, and for a large, old dragon, maybe only once a month.

As for the fire breathing, digestion produces methane, so dragons might have a special bladder where they can store methane. All you need is to belch out that methane and a little spark and you've got a natural flame thrower. Striking a couple of rocks together should be enough to get you a spark to ignite the methane. Or maybe dragons grow special teeth that spark when struck together.

Dragon bones are going to need to be very light, like bird bones. And that armored skin is going to need to be something light weight, not thick and heavy like a crocodile. I dunno, maybe make the armor out of same thing as space shuttle tiles, but grow it organically. I don't know how you could grow such tiles, but I don't know how we grow teeth either. In any case, dragons are totally possible. Nobody is going to create one any time soon, but maybe in a thousand years we will have learned enough biology that we would be able to. Then we could have the nine zillionth season of Game of Thrones, but for real.

Update - Stu did some math on this.