I recently visited the waterhole at Erindi Private Game Park in Namibia. I had the opportunity of a lifetime witnessing an elephant bull attacking a fully grown hippo with her calf.
The hippos were being fed grass by the park staff, I assume due to the present drought in Namibia. The bloat of hippos were happily grazing on the fodder when an old elephant bull came charging towards them and also started grazing. A mother hippo and her calf ventured too close to the grumpy bull and he attacked the mother, flipping her over with his trunk. She fell on her back with a massive thump and rolled once before getting back to her feet and retreating into the water. The elephant stood there for a few seconds taken back at what he had done, it almost seemed like he felt bad. He soon continued to mock charge the rest of the hippos, causing havoc. Soon the hippos showed aggressive behavior towards each other. After grazing, the elephant went to the waterhole making the crocodiles scatter. The mother only sustained a minor scratch but obviously a bruised ego.