Spanking The Monkey
This easter egg is found in Season 2, Episode 23b titled, Gutterballs.
In the scene where Mr. Big Head asks Rocko if he and his friends (Heffer and Filbert) would like to play a game (bowling), Rocko responds stating, “We’re already playing a game”.
The game in question that Rocko, Heffer and Filbert are playing turns out to be “Spank the Monkey”, a game which involves spanking a bare butt monkey so it turns around on the perch its sitting on.
However harmless the game may seem, “spanking the monkey” is actually a well known slang term for masturbation.