I'm in the process of creating flying creatures big enough to be mounted, but there's a problem: the bigger the size, the heavier the weight, the heavier the weight, the more difficult (or even impossible) it is to fly. And some of my creatures have 6 limbs. With that in mind I remembered that there are gases lighter than air and that birds have hollow bones filled with air pockets, so I got the idea that my creatures' hollow bones are filled with some gas lighter than their own. Can a creature fill bones with gas that is lighter than air?
It has to be a gas that the animal can get from nature or produce with its own body, ideally it is not toxic or flammable (imagine if someone shoots a fire arrow at a pegasus and it explodes). If you can, you can talk more than one gas (if they are gases that the animal produces with its body, then there is an alternative for carnivores and herbivores). My heaviest creature can weigh up to a maximum of 450kg.