Short answers, pulled from the story.
The hippopotamus is most closely related to cetaceans - whales, dolphins, and porpoises. The two lineages diverged roughly 55 million years ago. This relationship was not established until evidence from blood proteins and molecular DNA analysis overturned the earlier classification that grouped hippos with pigs.
The hippopotamus is considered one of the most dangerous animals in the world due to its aggressive and unpredictable nature. Hippos can capsize small boats and kill passengers; a 2014 incident in Niger saw a hippo capsize a boat and kill 13 people. The pharaoh Menes, according to the Ptolemaic historian Manetho, was killed by a hippopotamus.
Hippo skin secretes a substance called "blood sweat" that is neither blood nor sweat. Researchers have identified two acidic pigments in it: red hipposudoric acid and orange norhipposudoric acid. These pigments inhibit bacterial growth and absorb ultraviolet light, functioning as a natural sunscreen.
Hippopotamus amphibius once ranged across Europe as far north as Stockton-on-Tees in Great Britain, as well as across the Arabian Peninsula and the Sahara during wetter periods. The youngest European records date to around 40,000-30,000 years ago from Greece and the Rhine Graben of southwest Germany. Archaeological evidence places hippos in the Levant as recently as 3,000 years ago.
The IUCN Red List classified the hippopotamus as vulnerable as of 2017, with a global population estimated at 115,000-130,000 animals. The species is listed under Appendix II of CITES, requiring documentation for all international trade. The most dramatic population decline occurred in Virunga National Park, where numbers fell from around 29,000 in the mid-1970s to 800-900 by 2005 due to the Second Congo War.
Obaysch was the first zoo hippopotamus in modern history, arriving at the London Zoo on the 25th of May 1850. He attracted up to 10,000 visitors a day and inspired a popular song, the "Hippopotamus Polka". The earliest recorded captive hippos date much further back, to 3500 BC in Hierakonpolis, Egypt.