My bet would go to the plankton fauna. They are pretty defenceless against any predators, such as fish, mollusks, crustaceans and cnidarians.
Too Weak to Support Its Own Body Weight: The Jellyfish
Almost every living creature on the planet has at least the strength to support its own body and move around. One exception to this is the jellyfish that is one of the weakest animals.
Why tardigrades are so damn tough. It's important to note: Tardigrades are basically indestructible only when they enter a special state called cryptobiosis. In harsh environments, the animals tuck in their legs and expel all moisture from their bodies. In this state, they're called tuns.
The most famous example is that of the dodo, which owed its extinction in a large part to a lack of fear of humans, and many species of penguin (which, although wary of sea predators, have no real land predators and therefore are very bold and curious towards humans).
For example insects, arachnids and crustaceans don't feel any type of emotion. They don't show any signs of fear or pain. This is just down to the fact that their brain is too simple to hold this information.
Animals with no natural predators are called apex predators, because they sit at the top (or apex) of the food chain. The list is indefinite, but it includes lions, grizzly bears, crocodiles, giant constrictor snakes, wolves, sharks, electric eels, giant jellyfish, killer whales, polar bears, and arguably, humans.
Mantis shrimp pack the strongest punch of any creature in the animal kingdom. Their club-like appendages accelerate faster than a bullet out of a gun and just one strike can knock the arm off a crab or break through a snail shell. These small but mighty crustaceans have been known to take on octopus and win.
The Scoop. The honey badger has been called the world's most fearless animal because it doesn't hesitate to attack animals much larger than itself- even lions and crocodiles! Honey badgers are found in arid grasslands and savannahs and even rainforests.
Anything that tries to kill a tardigrade will quickly realize they encountered practically immortal animals. These creatures are capable of sticking around for thousands of years or even indefinitely “by entering a state of cryptobiosis, whereby their metabolism comes to a halt,” says Pastor.
Considered the softest animal in the world, chinchillas have 80-100 hairs per hair follicle (hole that hair grows out of), where humans only have 1-2.
Humans may have big, bulbous brains, but when it comes to pure muscle power, we're often considered the weakest of the great apes. Even chimpanzees, who are significantly smaller than us, exhibit levels of strength that are practically super-human by our standards.
Panda Bears
These fluffy black and white bears are one of the most adorable mammals. For the most part, pandas are pretty easy going and just eat bamboo most of the day, however, if they feel threatened they can be extremely dangerous. They can use their physical strength, strong jaws, and teeth to do a lot of damage.
The quokka, a small marsupial native to Australia, is one such example of a species vulnerable to extinction in the country's harsh surroundings. Known as the “happiest animal in the world” due to its cute and friendly appearance, these creatures are now only found in a few isolated forests and small islands.
Sloths are solitary creatures who want to be left alone, thus unlike household animals, they do not like to be touched. So, if you come too close for their comfort, they can be deadly and severely hurt people.
Even though snakes can't feel real love or affection for their owners, this does not mean that they are bad pets. As long as you do not feel the need to bond deeply with your pet, you can get quite a bit of joy out of keeping a snake.
Skunks are shy and docile animals that are easily startled — and you definitely don't want to startle one. Preferring to to live alone, skunks are usually only social during mating season. Several females may also share a den during the colder months to stay warm.