How would they look like? Being more specific and less speculative: would higher atmospheric pressure (ex. 5 atm) mean that it is easier to fly/glide because lower wingspan would allow to achieve the same lift? Or it would be actually harder, because denser atmosphere would mean higher drag and flying would be actually more expensive in terms of calories? Assuming that the composition or atmosphere would be roughly comparable to Earth's with higher pressure that would mean proportionally more oxygen so more primitive breathing and blood system could be used. (or an advanced one could be much more efficient) Which would allow to burn calories quicker. Am I right? Assumptions: carbon chauvinism, so on molecular level life roughly similar to that known from Earth.