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 captured in the wild, and its Pokédex entry was 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.

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 the glitch Pokémon is sent to the Pokémon storage system, it will beome 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 This Pokémon can be stabilized if it is deposited and withdrawn from the Pokémon Day Care on Route 5.