The 3.5 ton hippopotamus lived in South Africa's Kruger National Park and was being experimented on by Dr Markus Hofmeyr with a new kind of sedative for animals.
reported that the tranquilizer the vets normally used was too strong for hippos because it was made for elephants and rhinos. The hippos would often drown with that sedative because their heads would fall under the water.
According to The Sun
, Dr Hofmeyr's colleague Dr Nico de Bruin pulled him out of the way of the angry hippo after it lunged at the men and knocked Hofmeyr into the river.
"The animal snapped in our direction and came for us," Dr Hofmeyr told Sky News
A gamekeeper shot and killed the hippo when it went after the men again as they were scrambling to get away.
Dr Hofmeyr said: "We think the hippo was woken by water up its nose. We'll muzzle these animals in future."
Amazing photos of the hippo attack can be seen here.