This creature is a radially symmetrical flying creature called a streya. The adult streya has eight flat-ish legs surrounding a body shaped like an Mento or M&M. It attains flight by running in a circle in place, effectively spinning on its vertical axis. As it gains rotational speed, the streya lifts opposing pairs of legs until all eight are in the air. The assumption is that it gains lift much the same way a frisbee or helicopter does, and alters its speed and direction by means of tilting its legs/paddles/propellors.
When the streya is newly born, it doesn't have any legs, and is essentially immotile and helpless, not unlike a newborn human (except, you know, it's one foot wide and kind of hard-shelled and shaped like an M&M). The only defense mechanism I want to give a newborn streya is the ability to float. Streya have internal organs that generate a lighter-than-air gas and will allow an infant streya to float, but have effectively no control over its direction and speed. It will need to depend on its parent for rescue.
Here are the questions:
1. What plausible biochemical process can generate a lighter-than-air gas buoyant enough to lift a creature that weighs no more than ten pounds? My theory here is that an internal organ converts external air into a lighter-than-air gas, and the gas fills bladders within the shell of the infant streya like a tiny dirigible. Assume an Earthlike atmosphere.
What gas would work best? Hydrogen's flammable as hell, would make things interesting, and it might be plausible for a creature to generate it organically. I don't know how an organic creature in an Earth-like environment could produce helium. Methane is definitely plausible. Ammonia less so, unless it's partially feline (ergh, reminds me, I need to change the litter pans).
2. An adult streya needs to be able to stay in the air a good long time to hunt and migrate. It needs to keep spinning, needs to have some sort of engine to keep it turning. If it's airborne, it has nothing to anchor itself to in order to turn itself in place. And all frisbees fall down eventually. The best thing I can come up with for an "engine" is an organic gyroscope of sorts; internal musculature somehow keeps the whole creature spinning round and round. Am I thinking about this right? The exact details aren't terribly important, but if I can describe it as an "organic gyroscope" or something similar, that might be good enough.
3. What in the crap word is it I'm looking for when I want to describe a creature shaped like a Mento or M&M? Despite their ubiquity, there are no Mentos in this world. A dark, dark place indeed. "Lozenge shaped" might be right, or maybe a "circular ellipsoid"? O geometry nerds, help a brother out.