A rare black rhinoceros that's been at the Denver Zoo since 1984 and one of an estimated 3,700 in the world has died. Zookeepers and veterinary staff euthanized the rhino named George Tuesday after workers found it unable to get up. George suffered from a variety of health problems and was losing weight. A necropsy revealed that George had suffered a ruptured spleen. Born Jan. 11, 1983, George arrived in Denver in 1984 from the Cincinnati Zoo. George fathered a calf named Tony in 1992. The Denver Zoo also houses two other black rhinos."All of us at Denver Zoo will miss George greatly. He was a wonderful animal and a great ambassador for his wild brethren. Black rhinos are critically endangered in the wild and there are less than 90 black rhinos in North American zoos. A loss like this is felt among each zoo that has worked so hard to support the conservation of this animal. It is a tragic loss that is felt by many," said Denver Zoo spokesperson Tiffany Barnhart. Zoo officials said black rhino horns are sought after as medicine and dagger handles in some countries. The International Union of Conservation of Nature says black rhinos have dropped in numbers from about 100,000 in 1970.