There is also a way to exploit the Johto Guard so you can get 2nd gen Pokemon into 1st gen.
The
Johto guard glitch is a
glitch in
Gold, Silver, and
Crystal which allows players to trade a
Generation II Pokémon into a
Generation I game as an equivalent
glitch Pokémon or Generation I Pokémon by using a
????? (index number 255).
A ????? (index number 255) can be obtained by using the
Celebi egg glitch and having the
Pokémon Day Care on
Route 34 raise an obtained
????? (index number 000) 'bad clone'. After withdrawing it, the ????? (index number 000) 'reverts' back into a ????? (index number 255).
This glitch takes advantage of ????? (index number 255)'s 'cloaking' abilities. If the ????? (index number 255) is switched above a
Generation II Pokémon, it is possible to bring Generation II Pokémon into the
Time Capsule, as long as each Generation II Pokémon in the party is hidden below the ????? because Pokémon below it are temporarily regarded as being of index number 000 and ????? (index number 000) is not considered as a Generation II Pokémon by the game. As such, Pokémon below the ????? (index number 255) will be regarded as an
'M in
Red and Blue, or a
3TrainerPoké in
Pokémon Yellow.
However, the game will still block the Pokémon if it has any
moves which originated from Generation II even if ????? (index number 255) is above it, so any moves which originated from Generation II must still be removed by the
Move Deleter if they are known by a Pokémon to be hidden by the ????? (index number 255).
If the ????? (index number 255) is switched back underneath the Generation II Pokémon after entering the
Time Capsule, its original species is restored, meaning that Generation II Pokémon can be taken into the Time Capsule. Alternatively, the 'M or 3TrainerPoké can be taken into the Pokémon Day Care on
Route 5 where it will revert back into the equivalent Pokémon.
Since Generation II Pokémon do not exist in Generation I, the received Generation I Pokémon is determined by a conversion table for index numbers 1-250 which is hard-coded into the ROM of
Gold, Silver, and
Crystal, as can be found
here. It is currently unknown what determines the equivalent Pokémon in Generation I for a Generation II Pokémon with index numbers 000 and 251-255 because the equivalent Generation I Pokémon is no longer consistent on the species for these Pokémon.
Thanks to Bulbapedia for all the info. Link is here:
https://bulbapedia.bulbagarden.net/wiki/Johto_guard_glitch