Slap Happy Lion is one of Tex Avery’s reaction cartoons. The lion reacts to its own roar, the lion reacts to the mouse, the animals react to the lion. It doesn’t work as well as Northwest Hounded Police, which was the exact same type of picture. In that one, a bad guy is clearly established at the outset, so he deserves all the things Droopy does to him which brings about the outrageous takes. But in this cartoon, the lion doesn’t deserve anything; he’s not a villain. And who feels anything for the mouse? He neither heroic nor a victim.
But back to the gags. The lion’s roar scares every creature in the forest (except the mouse), and Avery and writer Heck Allen come up with reaction gags. A kangaroo (yes, in Africa) is frightened, looks for a place to hide and jumps in the pouch of another kangaroo who... well, it ends with the final kangaroo not seeing a place to hide—so it jumps into its own pouch and disappears.
Ray Abrams, Walt Clinton and Bob Bentley are the credited animators.