Onchoichthys agassizii(Louis Agassiz's Master fish)/Obese koi fish  https://images.squarespace-cdn.com/content/v1/4f6c8d6ae4b08696a7443c8b/1378690543970-APJK6ISVFMN1233MTGAC/Screen+shot+2013-09-08+at+8.28.25+PM.png?format=750w Named for the famous Swiss ichthyologist, this was a giant of a fish. At 27.3 tonnes and measuring 10 metres long, it has gained the jocular nickname 'obese koi fish.' Onchoichthys was a cosmopolitan species, traveling around the globe to lay millions of eggs in schools of hundreds. They first appeared in the Late Eocene and went extinct in the middle Miocene, though some remains suggest a Pleistocene extinction instead. It's possible these fish were outcompeted by the rorqual whales(e.g. fin whales, blue whales, minke whales...) Onchoichthys was a filter feeder, unlike its modern relatives, feeding on plankton. In turn, the large shark Megalodon and raptorial whales were a threat. Juveniles would be vulnerable to those as well as seals, sea lions, smaller macropredatory sharks, whales and pelagorns. In response to the attack strategy of 'ambush from underneath' tactic, Onchoichthys developed osteoderms on its body and a row of spear-shaped spines for defense. Unfortunately, they also disrupted the hydrodynamics which could be a facet of its extinction.