You've triggered the Rhydon glitch. After viewing the Pokedex entry of a Pokemon with an invalid Pokedex number (higher than 152 or 000) and viewing the Pokedex entry for that Pokemon, a subroutine is called that automatically turns the Pokemon into a Rhydon. If it's caught with a full party and sent to the PC, it will become an unstable hybrid of the original Pokemon and Rhydon. However, by simply catching the glitch Pokemon a second time or activating the flag for its Pokedex. For example, Pokemon with a Pokedex number of 000 like MissingNo. will not show their entries if they already captured a Cubone. The Rhydon glitch doesn't trigger on Pokemon that share a valid dex entry, or through trade or evolution.
If you capture MissingNo. again, it shouldn't display its dex entry and will be fine.
Source: the Bulbapedia article on the glitch has more information.