Lion is a Game & Watch game released in 1981. It is referenced by Mr. Game & Watch's forward tilt, as a stage transformation of Flat Zone 2 and Flat Zone X, as a sticker, as a trophy, and as a spirit.
As a stage
In Lion, the aim of the game is for two players on either side of a cage to keep a lion from escaping by using chairs, making the game very similar to Pong. As the game progressed, multiple lions would be in the cage. With this inspiration, Lion is incorporated as one of the changing stages played within the Game & Watch system in Flat Zone 2 (along with Fire, the default; Chef; and Oil Panic). The lion cage would encompass most of the stage, with two Lion Tamers on either side, attempting to repel the brawlers back into the cage. The Lion Tamers' chairs both have massive knockback, and it is not uncommon for players to be juggled between the two. Thus, it is strategic for a player to attempt to throw or knock their opponent into one of the Lion Tamers for easy KOs (which is made easier by the notoriously close blast lines).
Lion appears as a sticker in Super Smash Bros. Brawl, exclusively usable by Mr. Game & Watch.