The levitate spell description states:
One creature or object you can see rises vertically, up to 20 feet, and remains suspended for the duration. The spell can levitate up to 500 pounds. An unwilling creature that succeeds on a Constitution save is unaffected.
The target can move only by pushing or pulling against a fixed surface, which allows it to move as if it were climbing. It can also move up or down as part of its movement. You can change the target's altitude by up to 20 feet on your turn. Otherwise, you can use your action to move the target, but only within the spell's range.
What happens to a flying creature if levitated?
It sounds like the targeted levitating creature would need to push/pull a fixed surface to move, but this doesn't make sense to me for a creature that is already flying.