Poison dart frogs
Bright colors in nature are often a sign of danger, and these flamboyant little frogs are no exception. The potent toxin secreted from their skin has been used by indigenous peoples in Central and South America to poison the tips of blowdarts.
The most deadly species, the golden poison frog, carries enough toxin on average to kill 10 to 20 men. That's impressive for a creature that only grows to about 1.5 inches in size.