Rescue Persons of Interest and Prisoners of War in Metal Gear Solid 5: Ground Zeroes and upload that saved game file. Download that saved game file from the main menu in Metal Gear Solid 5: The Phantom Pain to get various rewards. The people you rescued will appear at Mother Base as one of your rewards. The people that will appear are as follows.

Assassin Harrier: Appears after two of either Missions 7, 8, or 9.
Bitter Centipede: Appears following Mission 12.
Blue Chameleon: Appears following Mission 6.
Crying Sturgeon: Appears after two of either Missions 7, 8, or 9.
Eye: Appears following Mission 6.
Finger: Appears following Mission 6.
Frigid Mongoose: Appears after two of either Missions 7, 8, or 9.
Gray Wallaby: Appears following Mission 6.
Grizzly Hedgehog: Appears following Missions 3, 4, and 5.
Hideo: Diplomat who appears following Missions 3, 4, and 5.
Hungry Crocodile: Appears following Missions 3, 4, and 5.
Hunting Stallion: Appears after two of either Missions 7, 8, or 9.
Mad Wallaby: Appears after two of either Missions 7, 8, or 9.
Midnight Mastiff: Appears following Mission 6.
Night Tree Frog: Appears following Mission 12.
Ochre Chameleon: Appears following Missions 3, 4, and 5.
Pirate Capybara: Appears following Mission 12.
Roaring Capybara: Appears following Mission 12.
Wild Harrier: Appears following Missions 3, 4, and 5.
Wild Stallion: Appears after two of either Missions 7, 8, or 9.