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Aepycamelus -

During the Miocene epoch, around 15 million years ago, ancient ancestors of camels were a common sight in North America. This species looked more giraffe than camel, and was 10 feet (3 m) tall just at the shoulder.

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Andrewsarchus -

The largest predatory land mammal that has ever been discovered, the Andrewsarchus was a fearsome creature that roamed the Mongolian steppes during the Eocene epoch, around 40 million years ago.

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Arsinoitherium -

The Arsinoitherium, named after the mythical Egyptian queen Arsenoe, was native to North Africa during the early Oligocene epoch about 30 million years ago. Unique in its era for its two massive, rhinoceros-like tusks protruding from its snout, they are thought to have had a mostly herbivorous diet, despite their menacing look.

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Chalicotherium -

This massive creature inhabited Europe and Asia around 16 million years ago, although fossils have been found from as recently as 3.5 million years ago, suggesting they were quite a successful species. These distant relatives of sloths and bears spent most of their time on their haunches, gathering vegetation with their long front arms.

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Daeodon -

A distant, vicious, and massive ancestor of modern wild boars, the Daeodon roamed North America around 23 million years ago. It is thought to have had a ridgeline of hair along its spine, similar to wild boars, and was about the size of a modern rhinoceros.

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Doedicurus -

These massive prehistoric armadillos are considered an evolutionary stepping stone between dinosaurs and more modern animals. Similar to dinosaurs like the stegosaurus, it had a spiked ball on the end of its tail that was capable of dealing serious damage to predators and prey alike.

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Elasmotherium -

Some 39,000 years go, this relative of the modern rhinoceros, affectionately nicknamed the Siberian unicorn, roamed the Eurasian steppes leading a mostly peaceful life of grazing on grasses, apparently rarely having to use its gargantuan horn.

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Uintatherium -

This two-ton native of North America from 40 million years ago was similar, although much larger, to the modern rhinoceros, except for the distinctive difference of having not one or two, but six horns protruding from its head and snout.

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Megatherium -

One of the most famous megafauna of them all is the Megatherium, or giant ground sloth. Native to the woodlands of South America about 10,000 years ago, these direct ancestors of modern sloths were the size of elephants.

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Woolly mammoth -

The wooly mammoth, thought of around the world as the fuzzy ancestor of modern elephants, was one of the few megafauna equipped to survive the various ice ages that occurred over the epochs. In fact, the youngest woolly mammoth fossils discovered aren't even 10,000 years old.

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High Arctic camel -

Another direct ancestor of the modern camel that was endemic to North America was the High Arctic camel. In stark contrast to their desert-dwelling descendants, these camels preferred the boreal forests within the Arctic Circle.

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Irish elk -

Related to modern red deer, these massive creatures, with antlers that could spread out up to 12 feet (3.6 m), are referred to as Irish elk because the vast majority of remains have come from the bogs of Ireland.

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Arctodus -

As late as two million years ago, the Arctodus, described as a "grizzly bear on stilts," hunted amongst the forests of North America. With legs about one third longer than that of most modern bears, it is thought these ancient bears could easily outrun the native North American horses that existed around the same time.

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Glyptodon -

These massive ancestors of the armadillo, native to all of the Americas, were likely one of the first megafauna to be entirely killed off by early humans. Not only was the docile Glyptodon easy to kill for its meat, there is evidence of early settlements in South America using their massive outer shells as shelter from the elements.

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Ceratogaulus -

The Ceratogaulus, also known as the horned gopher, is unique in rodents across history as the only one to develop a horn of such prominence. These gophers were perfectly average in every other way, and were only about a foot (30 cm) long.

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Eremotherium -

Another massive sloth of eons past, the Eremotherium roamed the plains of North and South America during the Pleistocene epoch, which began two million years ago. This giant sloth had a peculiar strictly-tree diet, and would break down tress with its massive clawed hands before consumption. These resilient mammals stuck around for millions of years, until finally being hunted to extinction by early humans.

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Indricotherium -

The Indricotherium holds the title of the largest mammal to ever walk the earth. Endemic to most of Asia, this herbivore was nearly twice as large as the largest modern elephant.

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Entelodon -

Also known as the "killer pig," the Entelodon was an omnivorous mammal related to the modern wild boar that was the size of a cow. The first signs of the modern boar's characteristic tusks can be seen in the mouths of Entelodons.

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Mesonyx -

Native to the plains of North America around 55 million years ago, the Mesonyx is old enough to lead a very fascinating lineage. While first thought to be an ancestor of canines and wolves, the Mesonyx existed so long ago that it is actually a closer relative to whales than it is to any canine.

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Metamynodon -

While assuming the Metamynodon was an ancestor of the hippopotamus would be a forgivable mistake, it is in fact a direct ancestor of the rhinoceros. The reason for its apparent relation to the hippo is something called convergent evolution, a phenomenon where two entirely different species may, for a time, evolve similar characteristics due to their shared environment.

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Nesodon -

Native to the South American forests of 26 million years ago, the Nesodon is one of many unfortunate species whose evolution stopped with them. Despite sharing similarities with other large, rough-skinned mammals, no descendants of the Nesodon have been identified.

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Argentavis -

The largest bird capable of flight in history, the Argentavis dominated the skies around 20 million years ago. With a wingspan of up to 24 feet (over 7 m), the Argentavis is thought to have used thermal currents to stay in the air.

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Megalania -

The Megalania was a massive lizard, and an ancestor of modern monitor lizards. Capable of growing up to 23 feet (7 m) in length and weighing more than 4,000 pounds (over 1,800 kg), it is also thought to be the largest venomous vertebrate in history.

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Livyatan -

The Livyatan was a vicious, gargantuan ancestor of the modern sperm whale. In an ancient ocean filled with predators such as Megalodon sharks, the Livyatan whale was still an apex predator, and didn't have a single natural predator. Its gaping mouth was equipped with razor sharp teeth measuring 1.2 feet (36.2 cm) in length, the largest biting teeth ever discovered, on land or in the sea.

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Saber-toothed tiger -

The most famous extinct megafauna of them all is without a doubt the fearsome saber-toothed tiger. Contrary to popular belief, these predators were only very distantly related to other felines.

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Ambulocetus -

The Ambulocetus may mark the evolutionary moment some mammals, called cetaceans, like whales and dolphins, made their transition into the water. As the ancient ancestor of these aquatic mammals, the Ambulocetus still had four limbs and was just as comfortable on land as it was in the water.

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Leedsichthys -

The ancient Leedsichthys fish, which swam in the seas while dinosaurs of the Jurassic period roamed on land 200 million years ago, are the largest ray-finned fish to have ever existed. They dwarfed even the infamous Megalodon sharks, with the average individual stretching 52 feet (16 m) long.

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Meganeura -

Some 300 million years ago, a distant, carnivorous ancestor of the modern dragonfly, called the Meganeura, flew across the old lands of Europe. Scientists believe that, due to a higher oxygen content in the air during the Carboniferous period, these insects were able to obtain wingspans as large as 28 inches (70 cm) long.

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Titanoboa -

The largest snake ever discovered, aptly named the Titanoboa, thrived during the late Paleocene epoch, around 60 million years ago. These amphibious snakes could grow up to a terrifying 47 feet (over 14 m) in length and weigh up to 2,500 pounds (1,135 kg).

Sources: (ThoughtCo) (Treehugger) (Bright Side)

See also: What do we actually know about dinosaurs?

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