From Bulbapedia, the community-driven Pokémon encyclopedia.
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).
Hiding Generation II Pokémon
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 (00) 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).
The glitch is useful for obtaining glitch Pokémon, and can be used to obtain almost any glitch Pokémon in Generation I, including glitch Pokémon previously unobtainable via the Ditto glitch and old man glitch because of a procedure where an equivalent Trainer class is fought if glitch Pokémon with index numbers of 200 and above are battled in the wild.
Reverting 'M (00)/3TrainerPoké $ into an equivalent species
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.
If the Pokémon is traded as an 'M (00) or 3TrainerPoké $, it is actually an unstable hybrid of 'M (00) or 3TrainerPoké $ and the actual glitch Pokémon, with the actual Pokémon as the recipient and the 'M (00) or 3TrainerPoké $ as the donor. To stabilize an unstable hybrid Pokémon, a Pokémon can be evolved (in the way the recipient would), traded to Pokémon Stadium 2, or deposited and withdrawn from Pokémon Day Care. A stabilized hybrid will be the species and level of the recipient Pokémon (or its evolved form if it was evolved), but have the stats, nickname, and moves of the donor. Once stabilized, the Pokémon can be traded to the Generation II games.
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. 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.
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Before a method was found of obtaining a ????? (index number 255) without cheating, the idea of using a ????? to bypass a Generation II Pokémon in to the Time Capsule was proposed on TRsRockin's forums, but was never fully investigated until Paco81's discovery of the Celebi Egg glitch on Glitch City Laboratories.
*Indicates glitches which have been officially acknowledged