Why does the sun look flattened near the horizon?

It's because of refraction. when we look at the sun at the time it's straight to the horizon, just above some height(for eg: a hill or a mountain), the air above that height is more denser. So when the ray of light coming from the base of the sun passes through that particular height, the ray gets refracted towards the normal and away from the normal respectively. This is why the sun looks flattened near the horizon.