Introduced in Generation II these Pokemon evolves based on the relation between several stats. As of Generation V only one species uses this method to evolve, let's check them out.
It was apparent in Generation I that Hitmonlee and Hitmonchan shared some kind of link. With the arrival of Generation II that link was revealed: they share a common pre-evolution: Tyrogue. This was one of the first branch evolutions in the game, but also one to use a unique evolution mechanic. The relation between Tyrogue's Attack and Defense will result in one of three Pokemon when leveled from level 20 and onwards. Here's a nifty and useful table.
|Tyrogue||When his||Evolves into|
|Attack > Defense|
|Attack = Defense|
|Attack < Defense|