From Glitch City Laboratories
"Wild appeared!" is the result of when the player enters battle before receiving their first Pokémon in Generation I. 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!".
When this happens in Generation I Pokémon games, a variation of the TMTRAINER effect happens, where the game may freeze or the player eventually defeats the opponent by using a Super Glitch move.