Meritons can only successfully evolve in the month of December. They can't at any other time of the year.
Meritons are the ultimate hunting foxes. To camouflage themselves during the winter months (with snow on the ground), they attach themselves to the similarly-colored bark from nearby trees. After its prey is within sight, they release from the tree and quickly execute their plan.
Meritons are most active in the winter months, despite the need for the same amount of food throughout all of the year. There must be something about the colder weather that makes them fiercer.