Wings
Of course, Insects got there first, and still do very well out of it.
One could argue whether there is a huge difference between the pterosaurs and the birds, but certainly it took a while to get the feathers right.
As always, there is more than one way to achieve an objective.
Bats, for example.
"Flying" fish, for another that didn't go all the way.
And in like vein, all the "flying" squirrels, possums, frogs, snakes, etc.