One of my favourite conservation stories is the black footed ferret. They're one of only 3 ferret species in the world (black footed, European polecat and Siberian polecat) and are the rarest mammal in North America (population currently at around 300 - 400).
Declared extinct several times in the 60s and 70s, a rancher rediscovered them after his dog killed one and brought it home. At that time there were around 130 black footed ferrets left in the wild, 18 of them were caught for breeding.
Thanks to that breeding programme over 2,300 kits have been released back into the wild. A core breeding population is still kept in captivity but slowly hopefully these guys will be plentiful across their historical range at some point in the future.