After defeating the Champion, if you take out Charizard, Aerodactyl or Dragonite from their Poké Balls (via the Party menu), you can fly very high off the ground. This allows you to fly past most obstacles, including fences, trainers, low-lying houses etc. However, you cannot fly past gatehouses.
In addition, you can now encounter wild Pokémon while flying up high. These include rare Pokémon like Charizard and Dragonite. Also, if you’ve encountered the Legendary Birds in their nests, there’s an extremely low chance of finding duplicates of them while flying.
Wild Pokémon - All open areas
|Fearow||Normal/Flying||In the sky||40 (Route 3, 4, 9, 10, 22, 23)|
|Spearow||Normal/Flying||In the sky||60 (Route 3, 4, 9, 10, 22, 23)|
|Pidgeot||Normal/Flying||In the sky||5 (all other areas)|
|Pidgeotto||Normal/Flying||In the sky||35 (all other areas)|
|Pidgey||Normal/Flying||In the sky||60 (all other areas)|
|Charizard||Fire/Flying||In the sky||Rare spawn|
|Dragonite||Dragon/Flying||In the sky||Rare spawn|
|Articuno||Ice/Flying||In the sky||Extremely rare spawn|
|Zapdos||Electric/Flying||In the sky||Extremely rare spawn|
|Moltres||Fire/Flying||In the sky||Extremely rare spawn|
The fully evolved Pokémon--Pidgeot, Charizard and Dragonite--are the most interesting here. Excluding the Legendary Birds of course, but if you can encounter them, that means you've already got them!