On Tuesday 10th September, nine black rhinoceroses arrived in Tanzania from Thaba Tholo Game Farm in South Africa in an effort to repopulate the Serengeti with the species. More information is included in the link below: https://phys.org/news/2019-09-black-rhinos-safrica-relocated-tanzania.html
They are not really returning, there is a small population already present in the Serengetiy, descended from animals that dispersed there from the Ngorongoro crater. Nevertheless this is good news, as the Serengeti ecosystem could hold a large sustainable Black rhino population, but it needs more founders for that. It is also a more difficult area for poachers as it is relatively heavily visited and well-trained rangers are present.
This is not the eastern section of the Serengeti and not physically part of the National Park, but the western section and a private reserve Sasakwa/Grumeti.