What animal has three eyes?

The tuatara, a reptile found in New Zealand, has a partial third eye, also known as a parietal eye, on top of its head. This eye has a retina, lens, cornea and nerves, but it isn't used for sight.

Scales grow over the third eye of the tuatara in its first six months. This prevents the eye from being used as a sight organ, but it is still light sensitive. The purpose of the third eye isn't well understood, but it may help the tuatara to regulate its body temperature, absorb ultraviolet radiation for vitamin D synthesis and regulate its circadian rhythm.