Pokémon GO Looks To Be Getting Generation II Content Soon
Lugia and Ho-Oh oh my!
Data mining has found that in the lastest Pokémon GO update, the names of the Generation II Pokémon have been added to the list of possible Pokémon appearing for the player. However, moveset data hasn't been added yet.
This means that Niantic probably plans to add Pokémon #152-#251 in an update in the near future, but they're not available yet. The Silph Road, in addition to the above data, have found quite a bit of other data as well. You can see the full list by visiting their site.