The Dire Wolf, shown here with a jaguar for size comparison, is believed to be the largest Canis (wolf) species to have ever lived. On average they weighed around 70 Kilos (154 pounds) with the largest specimens weighing up to 110 Kilos (240 pounds). They were far more stocky than the modern grey wolf but only a little taller at the shoulder. Their prey included Bison, Horse and on rare occasions, Mammoths. They went extinct around 10,000 years ago due to loss of prey species.
The Griffinfly was among the largest insect species ever with a wingspan of almost 30 inches. It is thought that this species was able to grow so large due to the higher atmospheric content of oxygen during the time period in where it lived. It was highly predaceous like other dragonfly species but due to its large toothed mandibles it was able to take on larger prey. It went extinct during the Permian-Triassic extinction event - along with 90% of all of earths animals.
The Titanaboa was the largest snake to have ever lived and possibly the largest non-aquatic carnivore to have lived during its time on earth. It weighed around 1,150 Kilos (2,500 pounds) and its length is estimated to be around 40 feet long. It hunted partially submerged in water in an ambush type fashion, striking then constricting similar to a common boa-constrictor of today. It preyed on large fish species and crocodiles. It went extinct to to environmental changes.
The Goliath Bird-Eater is the largest spider species alive today with large specimens reach almost 12 inches in diameter. They have been known to hunt hummingbirds which gave them their name, though only on rare occasions. They typically feed on insects and toads. Despite its intimidating size it poses little threat to humans with a bite reportedly being no more painful than a bee sting - still gave me the heebie-jeebies researching it.
The Paracer is an extinct genus of hornless rhino that lived around 25 million years ago. It is considered one of the largest terrestrial mammals to have ever existed. It weighed around 18,000 kilos ( 40,000 pounds). The shoulder height was about 4.8 metres (16 feet) with a 1.3 metre (4 Foot) neck on top of that. Due to its size it had few predators and ate a diet mainly of shrubs, leafs and grasses. Its extinction is still somewhat understood with no one reason being identified.
The Blue Whale is the largest animal to have ever lived on this planet. It's sheers size is often underestimated. With an average length of 30 metres (98 ft) and weight of 170-180 tonnes its not even close. It's heart alone is about the size of a small elephant. Along with the title of largest animal ever, they're also one of the loudest. They're able to communicate with other whales up to 1,500 kilometers - that's around the same distance as LA to Seattle. Its primary food source is krill, a small shrimp species and it can consume up to 4 tonnes of them per day. Blue whale calves when born are already among the largest creatures on earth and gain around 90 kilos (200 pounds) in its first year by feeding on its mothers milk. They have an average lifespan similar to that of humans at around 80-90 years. They're pretty awesome if you ask me.
The leopard seal Is the second largest seal just behind the elephant seal in terms of size. It is second only to the killer whale among the arctics top predators. With 1 inch long canines, this seal is famous for feading on a variety of penguin species as well as other smaller seal species. When hunting penguins, the leopard seal patrols the waters near the edges of the ice, almost completely submerged, waiting for the birds to enter the ocean. It kills the swimming bird by grabbing the feet, then shaking the penguin vigorously and beating its body against the surface of the water repeatedly until the penguin is dead - pretty damn metal.
The Giant Ground Sloth, Megatherium, was one of the largest land mammals known. Weighing up to 4,000 kilos (8,800 pounds) and measuring up to 6 meters (20 feet) in length from head to tail. It is the largest known ground sloth, as big as modern elephants, and would have only been exceeded in its time by a few species of mammoth. It ate a diet of shrubs and leafs and it had bony spurs along its back, neck and shoulders that acted like chain-mail to protect itslef from the predators of the time such as Saber-Toothed Cats and Humans - Yes, we hunted these animals. It also put its 20 inch long claws to good use when defending itself. Climate change and excessive hunting are attributed to its extinction around 10,000 years ago. Part 1 here - http://imgur.com/gallery/B0rWS p.s - If I've missed any of your favorite large animals, I'm sure you'll let us all know in the comments!