Actually every animal has a third eyelid. However, there are some animals that are more intense and optimal in using these eyelids. for example, eagles and owls, for example, they use their eyelids to protect their eyes when fighting or when they pounce on their prey. These animals that live in water such as fish, crocodiles, turtles, and frogs that use this third eyelid to protect their eyes while in the water. More or less functions for swimming goggles for them.
according to the theory of evolution, man himself also has remnants of this nictitans membrane. Try to see your own eyes in front of a mirror, notice that the edges of his eyes are close to the nose, there is a kind of white mebran there. . However, in humans themselves this membrane is no longer functioning as in animals.