technically, its cubone who's mother is dead, not marrowack. and Missingno so is a pokemon, but he's numberless (missing number) so the original series only has 151
MissingNo is NOT a pokemon, is a game error that appeared in pokemon gold and silver if I don't remember bad. It appeared because of the combination of 3 things (simplified):
1) Every spot where the character can be placed has a list of numbers (which the player can't see), which represent the pokemon that can appear and the probabilities.
2) There is an error managment tool in the language the game was programmed, called Exception Handler. It was programmed to create an object with the name of the error that appeared.
3) Where does MissingNo come from? There is a small line in the entire map that has numbers of probabilities of pokemon appearing but nothing in the "pokemon that can appear" list. That list is filled with "trash numbers". That line is the famous right side of the fire gym island. When you stand there and a pokemon appears, the program notices that there are no pokemon with the number that appears on the pokemon list (that one that was filled with trash info). So the program jumps to the error handle corresponding object, called MissingNo (of "missing number"), and god knows why, that object was not entirely good programmed and a bug appears, showing you a pokemon with the object name and somewhere info converted in image type that shows up as MissingNos picture. There are also small chances of MissingNo appearing as a rare ghost and a tranfe aerodactil or kabutops. Those are 3 images that the creators could use in the game but forgot to delete xD
As MissingNo is a game error, catching it can make anything happen, as converting it into a Rydon lvl 161 (I saw that) or any game errors.
Random fact: The mentiones trash info is generated with the name change between you and "Old Man" when he teaches you to catch a pokemon at the beginning of the game.
Conclution: MissingNo is NOT a pokemon.