Back between October 26th and November 1st, Niantic held a special Halloween event in Pokémon GO. During this event, players could find that Ghost-type Pokémon would appear at an increased rate in addition to getting a bonus of candies for catching and hatching Pokémon.
In an interview with GamesRadar, Niantic has reported that, during the Halloween event, players caught a total of 1.3 billion Ghost-type Pokémon. Among those, Gastly was the number one most-caught, with Cubone the second-most and Drowzee the third-most.
Pokémon GO is available for iOS and Android devices.