A total of 23 additional Hoenn Pokémon are coming to Pokémon GO later today, January 23, including Flygon, Aggron, Cacnea, Camerupt, Lunatone, Solrock and much more. The official announcement from the Pokémon GO development team at Niantic is as follows:
Starting later today, 23 additional Pokémon originally discovered in the Hoenn region in the Pokémon Ruby and Pokémon Sapphire video games will begin appearing in Pokémon GO for the very first time. As you’re out adventuring with friends and family and exploring your local neighborhoods, be sure to share your favorite moments and photos using #PokemonGO on your social media channels.
—The Pokémon GO team
Source: Official Pokémon GO blog