"A cat caught a mouse."
Translation:Muson kaptis kato.
Because one word has an -n ending and the other one hasn't. So it is clear what catches what.
It's fairly common to do this in German (another language where you can do this) so I don't see why it would be uncommon here. (Although the sentence here could not be ordered like that in any case because both cat and mouse are feminine in German and the accusative/nominative for feminine nouns look the same)
So I guess "muson kaptis la kato" would be roughly equivalent to "A mouse (is what) the cat caught," in terms of moving something forward for emphasis.
I put "Kato kaptis muson" and that was rejected. I can't recall <subject> <verb> <object> getting rejected before. Why was it rejected now?