Pokemon FireRed and LeafGreen players can't catch any extra Eevees in the game, but that doesn't mean they need to trade to get the eeveelutions.
The lore of the Pokemon universe is vast, and as such, it's no surprise some Pokemon have been around since ancient times. These are the strongest.
There are 10 Pokemon in Pokemon FireRed and LeafGreen that players can only get by trading to trigger their evolution.