It is a cup for low level Pokémon between level 15 to level 20. The sum of the levels of the three Pokémon chosen cannot be more than 50. Technically, all Pokémon except for Mewtwo and Mew are eligible to enter (although only Mew is listed as prohibited in the in-game rules list). However, obtaining Pokémon at such low levels, especially evolved Pokémon, can get quite complicated. While some are catchable at low levels in certain versions (e.g. level 15 Dragonair in Pokémon Yellow), others are more complex and involve trading with in-game Trainers, then trading these Pokémon to different in-game Trainers in different games.
Catch a low level Golduck, then trade it for Rhydon at the Cinnabar Island labY. Trade the Rhydon for a Kangaskhan at the Cinnabar Island lab in the Japanese Blue, then trade the Kangaskhan for a Muk in the Cinnabar Island labY.
Ponyta, Seel, and Koffing are offered as rentals. While they are obtainable at level 15 with tradebacks, there is no other way to obtain them at any qualifying level. Ignoring tradebacks, their lowest possible levels are 28, 22, and 30, respectively.
Some of the rental Pokémon also feature illegal movesets.
Poliwrath knows Double Slap at level 15. This move is learned by itsPre-evolutions at levels 25 and 26 respectively, while the maximum level allowed is level 20, meaning Poliwrath can not use Double Slap in the cup without glitches or tradebacks.
Cloyster knows Aurora Beam, Clamp and Supersonic at level 15. These moves are learned by its Pre-evolution at level 30, 23 and 18 respectively. The maximum level allowed is level 20, meaning Cloyster can not use Aurora Beam or Clamp in the cup without glitches or tradebacks. It also cannot use Supersonic at level 15 without glitches or tradebacks.
Gyarados knows Hydro Pump, Leer, and Bite at level 15. While these moves are learned by Gyarados at level 1 in Red, Green, and Blue, it is not possible to catch a level 15 Gyarados in those games without glitches. A level 15 Gyarados caught in Yellow only comes with Tackle due to a change in learnset. Due to this, it is impossible for Gyarados to use Hydro Pump or Leer in the cup without glitches or tradebacks (with the latter requiring Pokémon Stadium 2's Move Relearner and the level 15 Gyarados on a Red, Green, or Blue cart). However, Bite can be used by a level 20 Gyarados, as it learns Bite at this level regardless of version.
Certain Pokémon are not offered as rentals, despite being available at a low enough level.