Planet of the apes is a planet populated by Apes who can walk and talk like humans. The head and vacuum handle of Mega Maid crash-land here after Spaceball I self-destructs. Two of the talking monkeys witness Dark Helmet, President Skroob, and Colonel Sandurz climb out of Mega Maid's nose. They seem none too happy to see the three Spaceballs, with one of them cursing, "Oh, shit. There goes the planet."
This sequence acts as a parody of Planet of the Apes. Considering the twist of that movie, it seems possible that this planet is actually Earth in the distant future.