Glitch Trainer
A Glitch Trainer refers to any trainer which was never coded into the Pokémon game but rather accessed through the usage of a game-altering device or a glitch. Glitch trainers usually have random Pokémon in their party, typically 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 team such as Missingno. and Charizard 'M, although surprsingly some glitch trainers can be encountered this way with classes which are never used 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. For example a Glitch Trainer exists occupying slot C8 (200) 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 this changes most of the player's Pokémon to level 153 Bulbasaurs knowing Explosion.