This one comes in the form of an expected recently extinctified (made up word) giant freshwater fish known as the Chinese paddlefish (Psephurus gladius). It used to haunt the mighty waters of the Yangtze River. These monsters measured in at around 21 feet long, but haven't been spotted in over six years, despite a recent survey conducted to determine their numbers, which look to be approaching zero.
If the Chinese paddlefish is indeed gone, like the Yangtze river dolphin before it, then the title of the world's largest freshwater fish will be bequeathed to another, like the Mekong giant catfish or the arapaima of the Amazon river.
But then, it's suspected that the paddlefish wasn't a true freshwater fish. Ready for new vocab word (for you non-ichthyologists)? Anadromous. That's when a fish lives most of its life in marine environments but then returns to freshwaters to spawn. It's thought that the Chinese paddlefish might have been (or is) anadromous.