"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