The yellow submarines are one of the most notable elements in the Rabbids series, as well as the most iconic props next to the cows and pink toilet paper rolls. The yellow submarines are a form of spaceship or UFO the Rabbids use to invade various locations such as Paris and so on. In the first Rayman Raving Rabbids, the submarines were mostly used underwater and fired missiles at Rayman. In later games, the submarines could fly and ocassionally transform themselves into robots a la Transformers.
In the original game, the submarines were piloted by Rabbids underwater. They launched missiles at Rayman and often released the Frog Rabbids (Rabbids wearing scuba masks). The submarines were designed to look like sharks, as the front of the submarines had red eyes and shark teeth painted on them.
In Raving Rabbids 2. the submarines were completely redesigned. They could now fly around freely through anywhere the Rabbids pleased. The submarines now had mouths painted on them, so they could look more like Rabbids. One of the submarines could transform itself into a large metal robot (an obvious parody of Transformers) and launch plungers and missiles at the player in the FPS mini-games.
Even though the submarines do not make a major appereance, they do make a cameo appreance in the mini-game Incredible But Stupid, where a Rabbid is surfing through space, sometimes a submarine can be seen floating around space.
In Rabbids: Alive and Kicking, the submarines are slightly redesigned, the mouths are now made of glass, allowing the Rabbids to see through it while they're piloting.
The yellow submarines are an obvious reference to a popular song performed by The Beatles.