Biggest

Land mammal: African elephants weigh up to 14,000 pounds and can eat as much as 600 pounds of food a day! Their trunks contain as many as 100,000 different muscles.

Marine mammal: The blue whale is the world's biggest animal, even larger than any known dinosaur. An average-sized adult is 80 feet long and weighs about 120 tons. This giant is also the loudest animal on Earth. Its call, which is louder than a jet, can be heard for hundreds of miles.

Bird: The ostrich can grow to 9 feet tall and weigh as much as 350 pounds. Its eggs, the largest of all birds, weigh up to three pounds. These flightless birds can sprint up to 43 miles per hour.

Fish: The whale shark , which is not a whale, reaches about 33 feet in length and weighs up to 60 tons. These fish prefer warm water and inhabit tropical seas. Don’t let their size scare you! They are tame creatures that have let swimmers ride on their backs.

Reptile: The saltwater crocodile can grow to 23 feet long and weigh up to 2,200 pounds. Often called “salties,” the ferocious reptiles are found in India, Southeast Asia, and northern Australia.

Insect: Giant walking sticks can grow to about 20 inches length. They protect themselves from predators by using camouflage to blend into their environments.

See also: Slideshow of the World's Largest Animals

Smallest

Land mammal: The bumblebee bat tips the scales at about 2 grams and measures between 30 and 40 millimeters as an adult. This tiny animal is found in Thailand and Myanmar.

Marine mammal: Sea otters weigh between 35 and 90 pounds. They sleep while floating on their backs on the surface of the water.

Bird: The hummingbird grows to only 2.5 inches long and weighs only 0.06 ounce. This tiny bird makes its humming sound by quickly beating its wings.

Fish: The Paedocypris progenetica, which was recently discovered in Indonesia, measures just over 1/3 of an inch long.

Reptile: The British Virgin Islands gecko is about ¾ of an inch long when full grown.

Insect: Fairyflies, members of the wasp family, are only about 1/5 of a millimeter long. They are so thin—about the size of a thread—that they can’t fly well and rely on the wind to carry them.

See also: Slideshow of the World's Smallest Animals

Fastest

Land mammal: The cheetah can run as fast as 70 m.p.h., and can go from 0 to 60 m.p.h. in just three seconds. These endangered animals only need to replenish themselves with water every three or four days.

Marine mammal: Killer whales , or orcas, can reach speeds of 48 m.p.h. Killer whales travel in pods, and each pod has its own unique sound. They are members of the dolphin family.

Bird: The peregrine falcon dives at speeds of up to 200 m.p.h. and can fly at a rate of 90 m.p.h., making it not only the fastest bird, but also the fastest animal. Ostriches can run as fast as 43 m.p.h.

Fish: The Indo-Pacific sailfish can swim at speeds of up to 68 m.p.h. The top jaw of this fish extends well beyond the lower, forming a long spear.

Reptile: The spiny-tailed iguana has been clocked in at 21 m.p.h.

Insect: Hawk moths can reach a speed of about 33 m.p.h.

See also: Slideshow of the World's Fastest Animals

Meanest

(Since many animals could claim the title “meanest,” 
this list is subjective rather than authoritative.)

Land mammal: The ratel, or honey badger, is one of the fiercest, most predatory animals on the planet. They prey upon many animals, including scorpions, porcupines, tortoises, crocodiles, and snakes. Attracted to honey, they also ravage beehives. Healthy ratels have no predators; their viciousness and loose skin, which make them difficult to grip, keep other animals at a distance.

Marine mammal: Leopard seals and killer whales are the most predatory of all marine mammals. Leopard seals have been known to seek out human victims. Killer whales are the only mammals that prey on leopard seals. Leopard seals, which are earless animals, inhabit Antarctic and sub-Antarctic waters.

Bird: The peregrine falcon, a fiercely aggressive hunter, preys on small mammals and other birds. It descends on its victims from the air at speeds of up to 200 m.p.h. Falcons don’t build nests. Instead, they lay their eggs on the ground, on the ledges of cliffs, or in abandoned nests of other birds.

Fish: The stonefish, which lives at the bottom of the ocean, is the most poisonous fish in the world. Its 13 dorsal spines release deadly venom, making it dangerous to both humans and marine life.

Reptile: One bite from the inland taipan, a shy snake contains enough toxin to kill about 100 people. This deadly snake is native to central Australia and lives in holes, feeding on small rodents and birds.

Insect: The anopheles mosquito is deadliest creature on Earth. It’s responsible for more than 300 million cases of malaria each year and causes between one and three million deaths.

See also: Slideshow of the World's Meanest Animals