I'm thinking about implementing a race of flying creatures in my current setting: an earth-like planet with a breathable (enough) atmosphere. The easiest way to do this is just creating a race of bird-analogs, but I stumbled across a specific picture, and I fell in love with the idea of having strange, almost sea-creature like beings that can inhabit the air.
(For anyone wondering, the art is made by Peter Ferguson, highly recommend all his other work.)
In my mind, the easiest way for these critters to keep themselves aloft would be to just "swim" through a very dense, or non-oxygen atmosphere. However, I wanted the planet to still be (somewhat) habitable for humans, and the atmosphere to still be breathable. I'm thinking about changing the atmosphere to one in which, the higher the altitude, the more pressure there is. That said, I'm not sure how, or even if, that's possible.
Note that, only the lower altitudes (maybe up to 1-3 kilometers above sea-level) need to be breathable for humans and at a pressure comparable to Earth's at the same altitude. So, if there's any sort of gas, chemical or some physical trick that I don't know of that could let creatures similar in shape to the ones pictured stay afloat above these heights, but would be deadly or dangerous for humans without any safety-equipment, I would take it. The biology of the creatures themselves isn't important, so don't worry about what they might need to breath or how any corrosive substances might affect their bodies. As an estimate, I'll say their body-density is 1kg/l.
I've laid out these things so you can factor them into an answer, and so that you know what my goal is to begin with.
To summarize my question more directly: Is it possible for a planet's atmosphere's density to somehow scale upwards with increasing height to support these creatures?