I'm in the process of creating flying animals big enough to carry a human being on their back, the problem is that if it's too big, the wingspan also has to be and muscle will be needed for the wings to function properly, but muscles add weight and a lot of weight can anchor the animal to the ground. So I've been thinking about bones being made of metal, there are metals (like iron) in animals, so the animal's organism could gather enough metal to gradually add metal to the bones (it would be like a hybrid skeleton, 80-90% normal and 20 -10% metal) as the animal grows. But metal is heavy and I don't know if this "metallurgy" is possible, so I'm looking for something more plausible. How to make the bones of a large flying animal hollow and at the same time strong?
Data about the animals I want to raise for my world: Pegasus: 21m wingspan, 1.8m high. Weighs 400kg. Dragon: 18m wingspan, 3.60m tall. 300kg (because it has 4 limbs, so it weighs less than a pegasus that has 6). Bird: 15m wingspan, 2.8m tall. 230kg.