In gen 1 if you loaded an invalid Pokemon you'd get a glitch Pokemon made from garbage code the game reads from where it shouldn't, or MISSINGNO. depending on its Pokedex number. Gen 1 is held together with chewing gum and prayers, and there's very little in the way of error handling. Later games would handle this by defaulting to a giant black and white question mark called ????? that would usually crash the game if you battled with it and could normally only be found using a GameShark.