Across a number of areas within the game, there will be various Pokemon that will appear that the Pokedex won't mention being found in that given area. These type of Pokemon Spawns are commonly referred to as Rare Spawns or Special Spawns, since the chances of them appearing is still quite low, and they can happen in both Wild Grasses and in the Sky when riding an airborne Pokemon. Most Rare Spawns happen to be Starter Pokemon; however, extremely rare Pokemon like Dragonite, Snorlax and Porygon can also be Rare Spawns. After defeating the Elite Four and capturing the Legendary Birds in their stationary locations, Articuno, Zapdos and Moltres will have the chance of appearing as Sky Rare Spawns as well.
Even though Rare Spawns in general have extremely low odds of appearing, there are a number of things trainers can do to increase said chances. The first thing one can do is use Lures, Super Lures and Max Lures, as each of these Items makes it more likely for a Rare Spawn to appear. By creating Catch Combos, players can also improve the odds of Rare Spawns appearing, having higher chances of doing so the larger the Catch Combo they achieve. Furthermore, Catch Combos can extend over multiple routes, and they don't have to pertain to the Rare Spawn you are trying to make appear. For example, you can create a massive Catch Combo of Caterpie in Viridian Forest and then go to Route 3 to have increased odds of finding Charmander as a Rare Spawn.
Rare Spawn Pokemon & Locations
Charmander: Routes 3-4 & Rock Tunnel
Charizard: Routes 1-4, 6-8, 10-25 (Sky)
Squirtle: Routes 24-25 & Seafoam Islands
Chansey: Routes 5-6, 8-9, 10-18, 21, 23, Diglett Cave, Mt. Moon, Pokemon Tower, Power Plant, Pokemon Mansion, Victory Road
Lapras: Routes 19-20
Snorlax:: Cerulean Cave
Articuno: Routes 1-4, 6-8, 10-25 (Sky)
Zapdos: Routes 1-4, 6-8, 10-25 (Sky)
Moltres: Routes 1-4, 6-8, 10-25 (Sky)
Dragonite: Routes 1-4, 6-8, 10-25 (Sky)