The third eye of animals is a real thing. It's actually the pineal gland, an evolutionarily old and structurally interesting part of the brain that is indeed sensitive to light. This photosensitivity is important to the functioning of the animal body, because the pineal gland controls production of melatonin, the sleep-inducing brain chemical that is important in regulating the circadian rhythm.
Some amphibians such as frogs and in the juvenile tuatara, have a parietal eye, a light-sensive structure that is just beneath the skin surface to receive light directly through a translucent membrane. That structure connects to the pineal gland and thus functions to regulate sleep cycles and hibernation directly according to ambient brightness.
In man, the pineal gland is a tiny thing, about the size of a grain of rice. It sits close to the middle of the brain, just above and behind the brain stem, snug between the two halves of the thalamus. So deep in the brain, it cannot be affected directly by light. Instead, it is stimulated indirectly by signals from the retina of the eye. This is why you can use brightness or darkness to help reset your biological clock when traveling across time zones. A unique thing about the pineal gland is that it is the only part of the brain that is normally calcified, at least in adults. That makes it appear as a bright, solid white spot in x-ray images of the brain.
If you are interested in the massive woo-woo that surrounds the idea of the third eye, a. k. a. the eye of Horus, the Internet will more than satisfy your curiosity or need to believe no matter how deep it may be. The third eye is majorly associated with the sixth chakra, also known as the anja (brow) chakra. The available instructions for 'activating' or 'awakening' the inner eye, mostly meditative practices, would fill a library of no small size. Theosophists, and the miscellaneous new-agers associate the third eye with an entire kit of woo, one that includes enlightenment, mental images of deep spiritual or psychological significance, inner realms of higher consciousness, and on and on. "The third eye is often credited for religious visions, clairvoyance, the ability to observe chakras and auras, precognition, and out-of-body experiences. People who are claimed to have the capacity to utilize their third eyes are sometimes known as seers."
third eye imagined
Stimulate your 6th chakra and feel the woo-woo
Secrets of the Third Eye