Rhydon trap

Error prevention and placeholders in the Pokémon games:

?????????? | ????? | ! | Abnormal Pokémon | Bad Egg | Enigma Berry (Generation III) | Error! | Error codes (Generations I and II) | Error traps for damaged save data | Error traps to prevent impossible progress | Event data debugging messages | Instant victory effect (Generation II) | Mystery Zone | N/A | Pokégear instruction booklet trap | Rhydon trap | Special (location) | This Game Pak is designed only for use on the Game Boy Color. | This Pokémon cannot be traded.

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The Rhydon trap is a error trap in Pokémon Red, Green, Blue, and Yellow.

Whenever a glitch Pokémon with a Pokédex number (family) greater than 151 is obtained and its Pokédex entry is registered, it will become a Rhydon if there is space in the party.

Avoiding the trap

The Rhydon trap can be avoided if the game considers the glitch Pokémon to have already been registered as being owned in the Pokédex.

The "own" glitch Pokédex flags for all glitch Pokémon are the "seen" Pokédex flags for valid Pokémon. For example, the "own" Pokédex flag for Missingno., 'M (00), g g and $ (No. 000 glitch Pokémon) is seeing Cubone; hence if the player has encountered Cubone, obtaining a Missingno. will not turn it into a Rhydon.

Additionally, in this example capturing the Missingno. would register Cubone as seen if it wasn't already; meaning that the player can avoid the trap by obtaining the same Pokémon twice.

Another means of avoiding the Rhydon trap is by obtaining an unstable hybrid that will become (a stable version of) the glitch Pokémon when it is received from the Day Care and stabilizing it. The action of withdrawing a Pokémon from the Day Care does not register the Pokémon in the Pokédex. It is possible to obtain such a Pokémon through the large storage box byte shift glitch.

The Pokémon merge glitch and PC Pokémon 9 hybridization side effect of the international 'dokokashira door glitch' may be used to obtain a Pokémon with a different or lesser ID of the original Pokémon, that is a hybrid between one Pokémon (the one that the player obtains) and the original. Since the recipient byte is unchanged, the only way to stabilize the Pokémon into the new Pokémon is to evolve it (or possibly use Pokémon Stadium 2).

With arbitrary code execution or a memory editor/cheating device, one can directly modify an address controlling an existing Pokémon species, such as D164 and D16B (D163 and D16A in Yellow) which control bytes 1 and 2 of the first party Pokémon species or DA81 and DA96 (DA80 and DA95 in Yellow) which control bytes 1 and 2 of the first stored Pokémon in the Pokémon Storage System species.

Status as a glitch

The Rhydon trap was programmed into the game as an intended feature, which may have been used for debugging purposes.

The following code in the Pokémon Red and Blue ROM is responsible for converting the glitch Pokémon into a Rhydon.

  and a              ; Is the dex number 000?
  pop hl
  jr z, .asm_13a7    ; If it is, it's invalid
  cp $98             ; Is the dex number less than 152?
  jr c, .asm_13ad    ; If it is, it's valid
Invalid:
  ld a, $1
  ld [$cf91], a      ; Set the index number to 01 (Rhydon)
  ret                ; Return as if nothing ever happened
Valid:
  push hl
  (...)

(Source: TheZZAZZGlitch)

Despite this, the Rhydon trap suffers from a glitch. If there was no space in the party before catching the glitch Pokémon and it is sent to the Pokémon Storage System, it will become an unstable hybrid Pokémon with Rhydon as the donor and the original glitch Pokémon as the recipient. This Pokémon can be stabilized if it is deposited and withdrawn from the Pokémon Day Care on Route 5.