Thankfully this was the name of the restaurant, not the fare that it served. The restaurant was a San Francisco icon for more than 30 years, closing its doors in 1987. Apparently, even though hippo was not on the menu, some of the recipes were a little “out there” as the saying goes. The Hippo Cook Book debuted in 1969.
Here’s a link to a bit of Hippopotamus Hamburger history: