How does Haunter evolve?

As of "Pokémon X" and "Pokémon Y," the only method of evolving Haunter is to trade it to another player. Once the other player receives Haunter, it evolves into Gengar. The other player can subsequently trade Gengar back to the original player. Haunter evolves from Ghastly at level 25.

Haunter is a ghost and poison-type Pokémon. It has a 50 percent chance of being either male or female. The Ghastly line of Pokémon is known for proficiency in special attack and speed. Haunter is described as a trickster within the Pokémon world, although it can often be malicious. Haunter's evolution, Gengar, has an additional form known as Mega Gengar.