When reading about the Pleistocene, one gets the impression that the fauna were in general more badass than they are today. The biggest bears walking the Earth to date are the Kodiak bear (Ursus arctos middendorffi) and the polar bear (Ursus maritimus), both of which can reach adult weights of about 550 kg (1,210 lb). They're pretty badass and they definitely would make off with more than just your pic-a-nic baskets. But not more than 12,500 years ago, there was a bear that could top 800 kg (1,760 lb) and stood twice as tall as a grown man when it reared up on its hind legs.
Ladies and gentlemen, prepare to reflexively evacuate your bowels in fear; I present to you Arctodus simus, otherwise known as the giant short-faced bear. It ranged all over North America in a giant swath extending from the northernmost reaches of present-day Alaska all the way down south to northern Mexico and Virginia (and perhaps even Florida).
Cladistically, its closest living relative is the relatively cute and cuddly spectacled bear (Tremarctos ornatus) of the Andes. An obligate carnivore, its ecological niche was much the same as that occupied by Tyrannosaurus rex 65 million years before: it was large and strong enough to take down even the largest prey, but in general, it most likely scavenged other predators' kills, using its enormous size to scare them off once the kill had been made.
Unlike T. rex, however, A. simus very likely interacted with another apex predator that had just begun to occupy North America: Homo sapiens. In a pattern that had been established with the advent of Homo erectus and Oldowan tool technology, H. sapiens was likely one of the contributing factors to the extinction of the giant short-faced bear, along with the two other familiar members of the great triumvirate of Holocene mass extinction: climate change and the concomitant extinction of much of its prey animals.
Since the giant short-faced bear was an obligate carnivore --- that is, it ate only meat --- it lacked the dietary flexibility of its chief competitor, who could eat anything in sight thanks to its recent mastery of fire and tool-making. Along with pretty much every other species of megafauna in the Americas, Arctodus simus went extinct scant millennia after the arrival of humans.