European bison

Europe's largest land mammal survived the early 20th century only in captivity. Breeders rebuilt it from that zoo and park stock, and from the 1950s bison were turned loose again, first in Poland's Białowieża Forest. That makes it the earliest return to the wild on this list, and one of the most complete. The population grew from about 1,800 in 2003 to more than 6,200 in 2019, and in 2020 the Red List moved it from Vulnerable to Near Threatened. The catch is fragmentation: of the free-ranging herds counted then, only eight were large enough to be genetically viable in the long term, and the species still depends on ongoing conservation measures.