Gravity does exist in this world, and so does air and clouds. But this creature only has enough weight to stay in the air without falling or being lifted up. It's big like a whale and it does have mass as in it takes much force to accelerate it in any direction. So it's like in outer space but it's on the planet. It doesn't need to worry about falling or flapping its wings to stay up and stuff like that. It might look different than birds.
I was thinking of giving it appendages that it can use to sail in the air like a boat but I don't know much about sailing and I don't know how that would work in 3D (how does it go up for example?) Although I know sailing can go windward. I know I'm already breaking the laws of physics here but I still want to make it's biology realistic. Maybe no propellers or stuff like that because we don't really see big animals in the real world having anything that rotates like a propeller. If the sail idea requires the creature to be too mechanical like something a man made machine can do then I don't want it. Is a sail viable? Do you have alternatives if it's not?