This big fellow is Arsinoitherium, a prehistoric swamp monster related to elephants and hyraxes. Those horns were once believed to be hollow – possibly used for making big, booming noises of one sort or another – but now seem to be thought to do basically what rhinos’ horns do – defend the rhino.
Unlike rhinos, these creatures’ horns are made of bone. They lived in northern Africa around 30 million years ago, when most of what’s now the Sahara was then a mangrove marsh.
Not having access (as far as we know) to a time machine, Heinrich Harder did the best he could with the few skeletons that were available around 1920, when he painted the noble beasts for a series of collector cards.