Glitch Trainer
A Glitch Trainer refers to any trainer which was never coded into a game but rather accessed through the usage of a game-altering device or a glitch. Glitch trainers usually have random Pokémon in their party, usually consisting of Pokémon with glitched stats and moves and the Pokémon are usually glitches themselves.
Many trainers occupying normal classes such as 'Cooltrainer Male' can be encountered using the Old Man Trick with Glitch Pokémon on their Pokémon such as Missingno. and Charizard 'M, although surprsingly some glitch trainers with classes which are never used in the game can appear this way such as Chief and Professor Oak.
Several other Glitch Trainers are only known to be encounterable through the use of the Ditto Trick presuming the player has not used an external device such as one with a variable name, first reported as 'Jacred'. Another glitch trainer causes the ZZAZZ glitch which replaces most bytes in-battle with an index number of 153, hence changing most of the player's Pokémon to level 153 Bulbasaurs knowing Explosion.