Leafeon was added in version 2.2.1 of Pixelmon. It evolves from Eevee when exposed to a Moss Rock, found in forests. To do this, level up an Eevee within 10 blocks of the Moss Rock and it should turn into a Leafeon. Leafeon's name comes from the words "Leaf", a type of foliage, and "Eon", a long period of time (Referring to evolution).
If you want a Leafeon, it is recommended you evolve your Eevee into Leafeon as soon as possible so it has the opportunity to learn Grass-type moves that may only be available at lower levels. In particular, the move Magical Leaf (a Grass-type attack with 60 power and 20 PP that never misses) can only be learned by Leafeon at level 21 and so it is highly advised that you turn your Eevee into a Leafeon before it reaches level 21. If you miss this move, your Leafeon can still learn Grass-type moves like Giga Drain (level 25, has 75 power and HP draining), Synthesis (level 33, heals half HP), and Leaf Blade (level 45, has 90 power). Leaf Blade is the final move Leafeon will learn from leveling up.