Tornelos/Voltolos: After obtaining all 8 Badges and heading towards Route 10, a lady in the gate will inform you of weird weather on Route 7. Go there, east of the Tower of Heaven and you will be pulled inside of a house there. An old lady tells you a story and, upon leaving the house, this Pokemon flies down before flying off.
You can run into this Pokemon at level 40 on any route with grass in it by walking into the tall grass while the weather is storming violently. (It is similar to "roaming Pokemon" from the previous generation.)