The mythological Cyclops most possibly originated from natives from Mediterranean islands finding the prehistoric remains of extinct dwarf elephants. The skull of these dwarf elephants was only twice the size of humans and their central nasal cavity was easily mistaken for a central eye socket.
2. The Dodo bird which has been long extinct was actually a giant pigeon. It was endemic to the island of Mauritius. It grew and lost its ability to fly due to Island Gigantism. It was first discovered by Dutch sailors. The word dodo loosely translates to "fat-arse" in Dutch.
3. Castoroides, also known as giant beavers used to roam North America during the last ice age. They were about 8-feet tall and weighed 130-220 pounds, about the size of a modern black bear.
4. Steller's Sea Cows were 30-feet-long Manatees and weighed perhaps 10 metric tons. It was discovered by a German naturalist in 1741. They were hunted to supply Russian seal hunters with prized meat on long sea journeys. Within 27 years of being found, they were extinct.
5. The dire wolf actually existed but went extinct 10,000 years ago. Their extinction has been related to their inability to compete for prey against faster wolves, making them scavengers.
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The extinct American cheetah could be the reason pronghorns run so fast today.
7. In 2012, remains of an enormous turtle, which was the size of a smart car, was unearthed in a Colombian coal mine. This gigantic turtle appeared 5 million years after dinosaurs went extinct. This prehistoric turtle had massive, powerful jaws that would have enabled it to eat anything nearby, from mollusks to smaller turtles or even crocodiles.
8. The African continent had a native species of bear known as the “The Atlas Bear” which suffered great population decline due to the Roman Empire's demand for animals for gladiatorial events. The animal was hunted to extinction by the 1870s.
9. All the domestic cattle today have descended from a single group of 80 Aurochs (a type of cattle hunted to extinction by early humans).
10. The Lions (Atlas Lion or Barbary Lion) used in the Colosseum became extinct in 1952. They were the largest lions to date and the only species of lions not to live in pride.
There was a breed of dog called the Turnspit Dog or Kitchen Dog that was bred by humans specifically to run on a wheel which turned the spit to cook the meat over an open flame. It is now extinct.
12. Following the extinction of the Tasmanian tiger in 1936, Tasmanian Devils are the largest carnivorous marsupial now on earth.
13. There were two species of frogs called Gastric-brooding frogs that swallowed its eggs, converted its stomach into a womb, and gave birth through its mouth. It went extinct in the mid-1980s. Scientists are trying to resurrect it.
14. The Elasmotherium, an extinct genus of rhinoceros is thought to be the origin of unicorn myths and legends.
15. The first animal to evolve vision was the trilobite, a distant extinct relative of spiders and shrimps.
There is an extinct, carnivorous cousin of koalas and wombats known as the Marsupial lion.
17. An elephant-sized sloth (Megatherium) existed and went extinct only 10,000 years ago. It was one of the largest land mammals, measuring up to 6m (20 foot) and weighing 4 tons.
18. There was a now-extinct species of crocodile that could gallop.
19. Ancient humans hunted Glyptodon, an extinct massive armadillo, in order to live within their shells.
20. The last Kauaʻi ʻōʻō (now extinct) was male. Its song was recorded in 1987. It was recorded singing a mating call, to a female that would never come. It died in 1987 as well.
A cave goat (Myotragus bakearicus) that went extinct approximately 5,000 years ago is the first known mammal to have become cold-blooded. Their bone growth rate is unlike any other mammal, and more similar to crocodiles in showing slow and adaptive rates to environmental temperature.
22. The Carolina Parakeet, a neotropical parrot that was native to much of the United States up until they were driven extinct by deforestation, hunting, and possibly disease in 1910. They lived in huge flocks of up to 300 birds and were likely poisonous to eat.
23. In 1875, a swarm of locusts (Rocky Mountain Locust) 1800 miles long and 110 miles wide swarmed through the western U.S., causing $200 million in crop damage. At 12.5 trillion insects, it was the largest concentration of animals ever seen. Less than 30 years later the species had vanished and was declared extinct in 1902.
24. The extinct American Lion that used to range all over the U.S. and into South America and was about 25% larger than today's African Lion.
25. The Alpine Spaniel, a now extinct dog bred by Augustinian Monks for rescuing travelers stuck in the snow of the Great St. Bernard pass. Dogs were sent in pairs so one could alert the monks when they found someone. They are said to be ancestors of modern-day St. Bernards.