Atmospheric beasts are often described as large, balloon-like organisms, whose bodies are filled with lighter than air gases. On Jupiter, where the atmosphere is mainly composed of hydrogen, the beast would need to have hot hydrogen inside, since there is no other lighter substance.
Not all atmospheric beasts are built like living balloons. Some are described as having tentacles or flipper-like organs. In some reports, they are actually cloud-like, being not completely solid, and able to change their density and their size.
A mysterious substance called star jelly, is often linked to atmospheric beasts. A famous idea is that when an atmospheric beast dies, it falls from the sky as star jelly, a smelly gelatinous mass that usually evaporates in less than a few hours.
Some believe that atmospheric beasts are Earth organisms, genetically related to the other lifeforms on our world. Others prefer to think that they are some sort of extraterrestrial being, sometimes thought to be space-borne creatures. Having originated on Earth or in space, atmospheric beasts are nevertheless largely thought to be non-sapient, and thus are viewed more as animals, than as aliens that we might be able to communicate with.