Colleagues at ZSL Whipsnade Zoo in central England found the senior zookeeper, in his 50s, immersed in water in the Asian rhino enclosure at 8.15 local time (08:15 GMT).
He was treated and kept warm by paramedics before being taken to Addenbrooke's Hospital in Cambridge.
"When we arrived it was clear that the patient had received a number of injuries, including injuries to the chest, abdomen and pelvis," explained Dave Tamarro, of the East of England Ambulance Service NHS Trust.
The zoo has launched an investigation, according to a zoo spokeswoman.
"Family and staff are with the injured man at hospital, where he is being well looked after," she added.