One of most endangered and unknown species (and coincidentally one of my favorite) is the saiga antelope.
In the Pleistocene (Ice Age) they were found in North America and up until the 17th-18th century they were found all the way from Romania to China. They're major threat is poaching for food and traditional Chinese medicine.
In order to protect this weird animal, I think it might be best if at least a smaller, managed population could be reintroduced to Alaska and Western Canada to increase their numbers. If the population takes off in NA, then they could import the animals back to the Caucasus and Mongolia. If the Eurasian population dies off then we'll at least have a living population.