Lunareon in Pokemon RB

Before Umbreon, there was Lunareon.

In Pokemon Red and Blue Versions, there was an additional Eeveelution hidden within the games. Programmers had originally planned this to evolve from Eevee with a Moon Stone, but they later decided to make things... interesting.

You must have Eevee, Nidoqueen, Nidoking, Clefable, and Wigglytuff in your party, at least one of which knows Dig. Also, Vaporeon, Flareon, and Jolteon must be in your Hall of Fame. Go to to Mt. Moon. Progress through the cave until you reach the spot where the Pokemon fossils once were. Stand in the spot where the Helix Fossil was, then use Dig. Rather than returning to the cave's entrance, you will fall down into another floor, Mt. Moon's hidden B3F. on this floor, there is only one room with no exits. Only wild Clefairy appear here. In the center of the room is a giant stone. Walk up to the stone and click A. A question mark will appear over the player's head and the message "Huh? Eevee is evolving!" will appear. The evolution animation will proceed. "Your Eevee evolved into Lunareon!" To escape the cave, use Dig again.

^Pure fiction