Eevee evolves into Leafeon when it is exposed to a Mossy Rock.
Leafeon is a Grass-type Pokémon introduced in Generation IV, first appearing in Pokémon Diamond and Pearl. In the original games, the player had to level up Eevee near a Moss Rock in a specific location. The rock functioned as an environmental evolution trigger rather than a conventional item such as a Fire Stone or Water Stone.
Leafeon's design combines Eevee's mammalian shape with leaves, plant-like ears, and a leafy tail. Its Pokédex descriptions often connect it with photosynthesis and the clean, fresh scent associated with vegetation. The evolution gives Eevee a useful Grass-type option alongside its many other branching forms.
A common source of confusion is Glaceon, which originally used a similar location-based method near an Ice Rock. Later games changed how these evolutions are obtained, and some versions use a Leaf Stone or other methods instead. The Mossy Rock clue still points unambiguously to Leafeon.