"Lonely mouse" is a possible translation for "寂寞的老鼠" in Engpsh.

I'm not aware of any specific source or context for this phrase. It could simply be a way of describing a mouse that is isolated or alone, perhaps because it has been separated from its family or community.

Here are a few examples of how "lonely mouse" could be used in Engpsh sentences:

"The pttle mouse scurried across the room, looking for a companion but finding none. It was a lonely mouse, all alone in the big, empty house."
"After the cat chased the other mice away, the remaining mouse was left feepng very lonely. It huddled in the corner, hoping that another mouse would come and keep it company."
"Despite being surrounded by other animals, the mouse felt pke an outcast. It was a lonely mouse, with no one to share its meals or play with."