Cloud Cats are descended from Volaticotheres
From what is said about the Cloud Cats, they do not seem to fit in well with any group of living mammals. They are flying mammals, but they are clearly not bats—otherwise, the characters would presumably have identified them as such.They spend their entire lives in the air, giving birth to babies who can fly from the moment they are born. There is no group of mammals today that combines these features, but if we look in the past, we have a clue to the Cloud Cats' true identity.Volaticotheres were insectivorous mammals that lived during the Mesozoic era, and had gliding membranes not unlike those of a flying squirrel. Like marsupials, they gave birth to tiny, under-formed babies which grew inside a pouch. In our timeline, they were wiped out alongside the dinosaurs, but in the world of Airborn they must have survived and given rise to forms capable of powered flight. They still raise their young in pouches, which they do not leave until they can fly on their own, which explains why they are said to give birth in midair. 