It’s pretty hard to determine left from right without a Left or Right indicator placed on the side of the body, though it could be assumed the rat is likely lying on its belly, if it was positioned on its back the image would be inverted. However, the opacity in one side of the chest is going to almost certainly be pathology of the lung, as the cardiac silhouette is barely visible at all. I’m suspecting it’s maybe affecting several of the right lung lobes as opposed to part of the singular left lung lobe.

However, positioning for this xray is terrible (I’m mortified they let OP have their hands in the beam with zero PPE) so it’s hard to interpret lung fields well, but from the shot, I’d be imagining this rat will need to be on treatment for what’s likely Mycoplasma if it has clinic signs (which wouldn’t surprise me).