The Portuguese Man O’ War is a siphonophore (or a colonial organism) that is often identified as a jellyfish (which could sting but is not nearly as deadly).

Their stinging venom is extremely painful for humans, and usually leaves red, large welts on the skin. Even worse: the venom can travel up to the lymph nodes, causing allergy-like symptoms. Fever and shock are also known symptoms. They are commonly spotted in the Indian, Atlantic, and Pacific Oceans.
If you’ve ever watched Shark Week, you know just how dangerous the Great White Sharks can be. They can be found all around the globe in coastal oceans and are responsible for the largest number of attacks on humans.

They don’t usually hunt humans for food, but it doesn’t stop them from biting humans when they come across them. Great White Sharks grow to immense sizes, and the females actually grow to be longer than the males, they can reach 20 feet and weigh up to 4,300 pounds.