Devolutionaries - S1.E34

Warp Darkmatter goes to the distant planet of Binipinardia to nab a naturally occurring de-evolving gas, guarded for eons by the local Binipinardians (...who happened to take a coffee-break at exactly the wrong time...). When Team Lightyear arrive on Binipinardia, they find the locals to be missing, and the only critters that seem to be around are some semi-sentient lemur-like things. XR is marketing his comic book, breakfast cereal, and adventure series at the time, so when the rest of Team Lightyear go off to look for the missing Binipinardians, he stays behind to look after the ship and to test out some product marketing research on the lemur-like things. Shortly after discovering that Warp Darkmatter has set up a pumping station, to ship the gas to Zurg, the rest of Team Lightyear gets hit by the de-evolving gas; Mira becomes ameboid, Buzz becomes neandertalic, and Booster becomes ankylosaurus-ish. When he finds out what has happened to his comrades (...and that the native Bin