What is an atmospheric beast?

it's a theorical kind of animal who lives floating in some high layer of the atmosphere, according to Karl Sagan it is possible for this kind of being to live in the atmosphere of gas giants like jupiter. they should have very low density or be like living balloons filled with light gases that allow them to float at high altitudes. some people guess that phenomenons like the so called "star jelly" could be remains of atmospheric beast that exists here on earth.