As of Monday, September 16, a multitude of new Pokémon, all straight out of Unova, are available on Pokémon GO. This list will be updated as the deployment progresses.
List of available Gen 5 Pokémon

Available in the wild and in 5km eggs.

Make Snivy evolve

Make Servine evolve

Available in the wild and in 5km eggs.

Make Tepig evolve

Make Pignite evolve

Available in the wild and in 5km eggs.

Make Oshawott evolve

Make Dewott evolve

Available in the wild, in Raids and 2km eggs.

Make Patrat evolve

Available in the wild, in Raids and 2km eggs.

Make Lillipup evolve

Make Herdier evolve

Available in the wild

Make Purrloin evolve

Available only in the wild in Asia-Pacific.

Make Pansage evolve with an Unova Stone

Available only in Europe, Middle East and Africa in the wild.

Make Pansear evolve with an Unova Stone

Available only in the Americas and Greenland in the wild.

Make Panpour evolve with an Unova Stone

Available in the wild and in 2km eggs

Make Pidove evolve

Make Tranquill evolve

Available in the wild and in 5km eggs

Make Blitzle evolve

Available in 5km eggs.

Make Drilbur evolve

Available in the wild

Make Yamask evolve

Available in 5km eggs.

Make Foongus evolve

Available in 10km eggs

Make Ferroseed evolve

Available only in Raids and 10km eggs

Make Klink evolve

Make Klang evolve

Available in 10km eggs

Make Litwick evolve

Make Lampent evolve with a Unova Stone

Available in 10km eggs

Make Golett evolve

Available only in the western hemisphere in the wild.

Available only in the Eastern Hemisphere in the wild.

Available in 10km eggs

Make Deino evolve

Make Zweilous evolve