What is the fastest animal on Earth? To answer that question, you have to be more specific. Are you talking about running on the ground, diving in the air in free fall, flapping through the air, or swimming through the water? Also, you might want to judge speed based on a creature's body size and how many body lengths it can achieve per second. This list includes the fastest birds, land mammals, fish, and even insects. You probably learned that the fastest animal on land is the cheetah, which is arguably correct... at least over short distances. But when you compare its top speed to that of a diving peregrine falcon or a black marlin in water, the cheetah's reputation for being the "fastest animal on Earth" seems a bit undeserved.Take a look at our list of speedy critters that can all fly, swim or run circles around you. Some of them may surprise you.