Leo is a full-sized Nubian lion (Felis leo) that washes up on the island after being lost at sea on its way from the Harris Expedition to the Singapore Zoo. Possibly washing up on the island as early as Gilligan's Mother-in-Law, the wooden crate that was transporting it washes up into the lagoon in Feed the Kitty. After possibly living off the indigenous birds and wildlife on the island, his roars alert the castaways to his presence and they seek him out to trap him, but it is Gilligan, who captures him in the Howell's Hut and befriends him after moving a splinter (possibly from the crate) from his paw. Gilligan names him Leo after his zoological classification, but everyone else remains terrified of him. Gilligan keeps Leo's temper down by feeding him corned beef, but when it starts running down, everyone finally convinces him to cage him. However, the tide comes in and washes him out to sea where a Navy Destroyer rescues him and finds him in good shape but addicted to corned beef!
- Leo was played by a domesticated lion named Zamba; his trainer doubled for Gilligan in several scenes.
- According to Sherwood Schwartz, Bob Denver surprised Leo by jumping off the bed in the Howell's Hut, but since the bed was on wheels, Leo rolled off mark and just barely missed fatally mauling him.
- A fake lion can be seen in the cage as it floats off, possibly because Leo/Zamba would have over-turned it.