(No, there is no 'Ballonts I', but there is an entry on 'The Interplanetary Zoo III', and that could have been called 'Ballonts I').
I guess that ballonts could do with a bit more attention. Let's start with a small illustration of their physics. I made the point that ballonts work better in heavy gases, meaning that you need a smaller gasbag to lift a given mass in a heavy than in a light atmosphere. Or, you can lift a heavier load with the same volume of gasbag; same principle. So here is a mixomorph larva. Its gasbag, as well as the gas, are supplied by the parent. The larva merely floats suspended from the gasbag. There is complete painting of them, by the way). It floats around passively until it lands, and then the larva can wiggle around for some time to find a suitable spot to root in, and there it remains for the rest of its life. Of course only one of these sketches depicts the real larva: the middle one. The one with the spindly larva shows the consequences of a light atmosphere, and the other one is its hypothetical heavy air cousin.
Playing around with this principle results in a series of organisms from different planets. At the top right you will find a ballont from a light atmosphere. With its large gas bag it is reduced to floating at the mercy of the winds. It does not even have any limbs for transverse movement, as these would be futile. Going down and to the left the gasbag gets progressively smaller. These animals live in heavy, soupy atmospheres. Locomotory limbs, i.e., wings, begin to appear, and these progress from thin weak flaps to sturdier shapes. At the end, just above the gray line, the difference in density between the gas and a fluid isn't very large anymore, so the animal takes on some characteristics of aquatic animals, such as wings that are almost flippers.
The gray line is literally a watershed; below it, we are dealing with fluids, and the shift in density is so large that the gas bag can be extremely small. In fact, it is hidden inside the animal, and is now known as a swim bladder. Gasbag or swim bladder, wings or flippers, they are the same things, really.