Like Magby, Pichu, Smoochum and other Babies, it evolves into other established Pokémon - but instead of one linear evolution, there are a total of three available, one of which is a new Gen 2 type, Hitmontop.
While on the surface it sounds like the Eevee evolutions, how you get from Tyrogue to Hitmonchan, Hitmonlee and Hitmontop works a little differently and requires a dose of luck. http://blog.pokemonenergy.com
First, you need to get a Tyrogue by hatching a 5km Egg. If you're playing just after the update has gone live, remember that Gen 2 Pokémon will only appear in Eggs collected after the update has gone live, so keep walking and hatching until one emerges.
How to evolve Tyrogue into Hitmontop, Hitmonlee and Hitmonchan
Once you have a Tyrogue, how it evolves from into the three fighting types depends on its stats:
If Attack is higher than other stats, Tyrogue evolves into Hitmonlee
If Defence is higher than other stats, Tyrogue evolves into Hitmonc...