It happened in the Legion of Super-Heroes' third volume, #32 to be exact. Heroes from the 30th century had been kidnapped, depowered and kept prisoner on a planet. Heroes like Saturn Girl, Dream Girl, Atmos, and... Jon Arbuckle from Garfield?
I wonder what his powers are. And if he'd have any chance at all in a Legion try-out.
Would they drum him out as soon as he started having conversations with his imaginary cat?