A virtually extinct ancient European bison
The Wisent (Bison bonasus) is the European bison. Although the wisent is slightly smaller than the American bison, the two creatures are very similar in appearance, and can actually breed with each other quite easily. Despite the fact that the Wisent is the largest land mammal in Europe, few people today are even aware of its existence, because the animal has been on the verge of extinction for centuries. The Wisent should not be confused with the aurochs, the now-extinct wild ancestor of modern domesticated cattle.
Just as the American Bison once roamed the Great Plains in massive herds, huge populations of wisent once wandered across large swaths of Europe, from the Caucasus Mountains to the tip of the Iberian Peninsula, and the animal was frequently depicted in cave paintings. But human hunting took a huge toll, even in prehistoric times, such that by AD 1000 the wisent was extinct throughout Europe except in small patches in the Ardennes forest and the Western Caucasus. The last wisent in the Ardennes were killed in the 1300s, and the last wisent in the forests of Transylvania was killed in 1790.
By the 20th century, wisent only existed in the wild in royal reserves which had belonged to the Polish kings, having been protected by a 16th century decree from Polish King Sigismund the Old which had made killing a wisent punishable by death, and a few preserves in Russia where they had been similarly protected by the Tsars. Nevertheless, wisent continued to be killed by poachers. The last wild Polish wisent was poached in 1919, and the last wild wisent anywhere was poached in Russia in 1927, at which time only about 30 animals remained, all of them in zoos.
Realizing that the fate of the wisent hung in the balance, the zoos embarked on a cooperative breeding program in the 1930s, but had to do without the wisent in German zoos, which were used by Adolf Hitler as part of his plan to crossbreed a genetically superior supercow (I kid you not). By 1951, there were enough wisent to begin releasing them back into the wild, and today there are just over 3,000 wisent alive in the wild, spread out in small herds in protected forests in Poland, Lithuania, Belarus, Ukraine, Russia, and Kyrgyzstan. Nevertheless, the species remains critically endangered, not only because of its small numbers, but more crucially because all wisent today are descended from only 12 animals, leaving them profoundly vulnerable to disease and deformity. To make matters worse, although there were 5 bulls in the 12 animals, the bloodlines of three of the bulls have died out since 1951, meaning all male wisent today have one of only two Y chromosomes.