In The Emperor's New Groove, people that have been transformed into animals by Ysma's potions can still speak human language. None of the regular animals can do this. In the very beginning of the movie, a fly gets trapped in a spiderweb and screams "help me!" before it's eaten. Someone came up with the idea that the fly is the former emperor whom Ysma poisoned in her attempt to take the throne, and that's how someone as incompetent as Kuzco ended up in charge. It also fits in with her initial brainstorm of how to poison Kuzco, where she considered turning him into a flea.
The person who thought of this wrote to the writer of the movie about their theory, and essentially had it made canon because the writer liked it.