"Wild appeared!" is the result of when the player enters battle with zero or a specific number of Pokémon over 6 in Pokémon Red, Blue, and Yellow. The name originates from what text the game displays when the battle starts, though what it exactly shows has more spaces between "Wild" and "appeared!".
This glitch occurs due to a buffer overflow caused by the game attempting to write 'Poké Balls in belt' graphics data (buffer at $CEE9) for Pokémon beyond slot 6. The possible values are usually always one of the following.
$31 - No status ailment
$32 - Status ailment
$33 - Fainted
$34 - Empty
A more detailed explanation can be found on this forums post by TheZZAZZGlitch.
53+ Pokémon: Screen changes colour before players sends out their Pokémon
222+ Pokémon: Replaces the music with a sound and usually causes it to stop due to the corruption of the memory address CFC7 (CFC6 in Yellow).
220+ Pokémon: Makes the game freeze in Red and Blue after battle if CFC4 becomes an odd value (see CFC4 freeze). But no freeze occurs in Yellow.
239+ Pokémon: 'Mutate' the enemy Pokémon into either Golem, Missingno. (hex:32), Magmar, or Missingno. (hex:33).
Note: All above effects occur for 0 Pokémon (as this is interpreted by the game as 256 Pokémon).
See alsoInfinitely long enemy Pokémon name
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