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. However, getting strong Pokémon at that low of a level can get quite complicated. While some are catchable at low levels in certain versions (e.g. Level 15 Dragonair in Yellow Version), others are more complex and involve trading across the in-game trainers and the different versions. Coincidentally, many of these involve the Yellow version.
Ignoring glitches and tradebacks from Generation II, the following Pokémon may be legitimately obtained and entered into the cup.
A Pokémon with a * next to the version can only be obtained by trading across different versions, while a Pokémon with ** can only be obtained in the Japanese versions with the Japanese Blue.
Catch a low level SlowbroWrong template. See Template:Sup/doc., then trade it for Lickitung at the 17 gateWrong template. See Template:Sup/doc.. Trade Lickitung for Dugtrio at Route 11Wrong template. See Template:Sup/doc.
In the Japanese Blue, trade a Kadabra for a Graveler at the Cinnabar Island lab. An eligible Golem cannot be obtained in the international Generation I games.
Catch a low level Golduck, then trade it for Rhydon at the Cinnabar Island labWrong template. See Template:Sup/doc.. Trade the Rhydon for a Kangaskhan at the Cinnabar Island lab in the Japanese Blue, then trade the Rhydon for a Muk in the Cinnabar Island labWrong template. See Template:Sup/doc..
Catch a low level VenonatWrong template. See Template:Sup/doc., then trade it for a Tangela at the Cinnabar Island labWrong template. See Template:Sup/doc..
Catch a low level Golduck, then trade it for a Rhydon at the Cinnabar Island labWrong template. See Template:Sup/doc.. In the Japanese Blue, trade the Rhydon for a Kangaskhan at the Cinnabar Island lab.