This idea was what originally spawned Missingno. In the Red/Blue/Green versions, Marowak WAS originally supposed to evolve into Kangaskhan. But the evolution was cut last minute making Kangaskhan a stand-alone Pokemon. Due to laziness and a deadline, Gamefreak didn't get rid of the excess data which invoked an evolution into Kangaskhan but instead dumped on the shore of Cinnabar Island, in beliefs that no one would ever find it. The "old man showing you how to capture a Pokemon" tutorial causes Pokemon to appear more spontaneously than usual, which makes it possible to go into battle with the extra data; Missingno. Thats also why Missingno can evolve into Kangaskhan.
Defeat you shall feel from the master of Steel.