Although habitat loss is an important consideration, the main threat to rhinos is poaching. Rhino horn is a highly valued component of Traditional Chinese Medicine, practiced in China, Vietnam and other parts of East Asia. Hundreds of rhinos are killed illegally for their horns every year.
From the same website:
In the first week of December 2011, Karen Trendler of Working Wild / Rhino Calf Response Strategy EWT was called to the scene of a rhino poaching incident in Limpopo Province, South Africa.
A white rhino cow and her calf had been shot only 800 metres from a lodge in broad daylight. The cow had been shot four times and had died at the scene, her horn hacked from her face. Her 11-month-old calf had been shot in the chest but had managed to run from the poachers.
The calf was tracked, and was found the following morning. A vet treated her as best he could but she died eight hours later. A post-mortem showed that the bullet had broken three ribs and then ricocheted off the fourth into her lung, and then on into her liver, small intestine, stomach and finally into her large intestine. She must have been in terrible pain.
There is an article about this topic in this months "National Geographic", and the image what has been done to the poor rhino is so terrible that I cannot post it here.