Oh, I remember what Jeff's talking about.
Depending on how the battle goes during your first encounter and the encounter on route... 22? The one west from Viridian City, in any case, you can manipulate how Gary evolves his Eevee. Losing twice means Vaporeon, Winning once and losing once or skipping the fight means Flareon, and winning both times turns it into Jolteon.
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My Little Phineas and Ferb: Summer is Magic!
Aww, I lost to SuperNiceDog, Winner of the Rivalry Rumble Guru Contest