These creatures spend their whole live in the air - Never ever landing, not even for breeding. Due to that the creatures actually developed an interesting kind of behavior. They travel only in very large swarms, hunting other flying creatures.
In order to rest, the creature have a special normally completely flat organ at the end of their tale. This organ is able to produce a special kind of gas, lighter than air, which enables them to float in the air without effort. This bloated version also encouraged building a special bond between two individuals among the swarm - one would usually hunt or move his or her mate to a new hunting ground, while the other one rest in the bloated version. The only reason they got still legs is actually to be able to grab onto a bloated individual.
They are live bearing and only have one or two children in one throw, but those develop fast. One of the parents stay in the bloated version until they can fully fly themselves.