Theory 1: Red convinced his rival Blue to trade his Blastoise away and his other rival Leaf to trade her Blastoise and probably scammed them with Promised Pokemon that were stronger than the kanto starters.
Theory 2: Red is a Pokemon yellow player, but than there would be a eeveelution on Blue's team, which there's not….
Theory 3: Red caught them in the wild… but that still doesn't explain where Blue's starter went, he either has a blastoise or he has a eeveelution, but in this case there's neither one.
by spectralcow1