I didn't use the game shark I did random things and in the viridian forest appeared missingno
You experienced the Rhydon Glitch.
After the Pokédex entry is displayed upon catching a Pokémon with an invalid Pokédex number, an affected Pokémon will become Rhydon, as a subroutine used when giving the player a Pokémon converts all invalid Pokémon to Rhydon. The Pokémon's moves will be replaced by Rhydon's moves at the level it was caught at. If this Pokémon is sent to Bill's PC, it will become an unstable hybrid Pokémon, with the original glitch Pokémon as the recipient and Rhydon as the donor. Due to all hybrid glitch Pokémon having Pokédex numbers equivalent to official Pokémon, they are not affected by this glitch.
Obtaining a glitch Pokémon with a Pokédex number less than 152 which is not 000. Obtaining a glitch Pokémon which doesn't show its Pokédex entry upon capture. Activating the flag which is the same as the obtained flag for the glitch Pokémon as a method of avoiding the Pokédex entry. For example, Pokédex entries belonging to glitch Pokémon in the No. 000 family (such as MissingNo.) will only appear upon capture if the player has not encountered Cubone. Obtaining a glitch Pokémon twice, as on the second time Pokédex entry flag will already be obtained and the conversion to Rhydon will no longer take place.
source
