4. Leafeon
Before the Poké-enthusiasts cry foul: yes, Leafeon is a real Pokémon. However, it was also only available from Generation IV onwards, meaning it made its first appearance long after the release of Pokémon Red and Blue. In a not-so-crazy chain of thought, many players assumed that Eevee could be evolved using a Leaf Stone in order to gain a grassy variant. It made sense, as a Water Stone led to Vaporeon, a Fire Stone led to Flareon, and a Thunder Stone led to Jolteon. Despite the Pokédex itself claiming that Eevee could evolve through exposure to elemental stones, it appeared that the Leaf Stone was the only exception. When Leafeon finally did come around, it was actually gained by levelling an Eevee near a Moss Rock, rather than using the (far more logical) Leaf Stone.