This is wrong. MissingNo is a specific Pokemon value holder for all numbers above 151, Gen 2, Pokemon exist, only a select few and they appear as MissingNo, This theory leads to some wacky misconceptions.
Now with the following...
Missingno
Here is one of the most interesting Pokémon Fan theories. The Missingno Theory.
Cubone was originally supposed to evolve Marowak. The Original was rewritten into what is not known as Pokémon # 000.
When a mother Kangaskhan dies, it leaves its Kangaskhan cub on its own to fend for itself. The Kangaskhan cub uses the skull of its mother to use as protection, becoming a Cubone. Then, later on, the Cubone evolves into a Marowak, and its mother’s skull fuses with that of the Marowak’s, and the bones become one. Female Marowaks (and only female) can later evolve into Kangaskhans for reproduction, and then the cycle starts all over again. This is self-evident, as a Cubone appears rather similar to the baby Kangaskhan that rides around in its mother’s pouch. Also, ‘Bonemerang’ (one of Cubone’s signature moves) refers to a boomerang just as ‘Kangaskhan’ refers to a kangaroo. Kangaroos and boomerangs are both natively Australian themes.
Before Pokémon Red/Blue released, the developers decided that Kangaskhan needed to be separated from the previous two stages to become its own standalone Pokémon. The reason for this is that, being a kids game, it wouldn’t do to allow small children to learn that a Cubone is actually an orphaned baby Kangaskhan wearing its late mother’s skull on its head and slinging its mother’s bones around as weapons. However, the team didn’t have time to delete the code at the last minute. Instead of deleting the code, they simply wrote a new non-evolving Marowak and moved the old one to an empty slot with no number. This catchable Pokémon is the infamous ‘M, better known as Missingno., meaning “missing number”. Missingno. is actually a scrapped prototype of Marowak. If you catch a Missingno. and level it up, it will eventually evolve into a Kangaskhan.