When French researchers released video of a gorilla going ape on his own reflection in a mirror, it went viral. But the gorilla's reaction is not that unusual for animals. In order to know that the reflection is not another animal, a creature has to be self- aware and there are not many species in the animal kingdom that have the ability to know they are looking at a reflection of themselves and not another animal.
In 1970, a test for self-awareness, using a mirror was developed. An animal or a human had a mark placed on their face with a smudge of body paint. When the animal was then placed in front of a mirror, those that knew it was a reflection of themselves would reach up to brush off the mark. Human babies develop this skill around the age of 18 months or so, and some animals have it, including some pigeons, elephants, dolphins and bonobos, but most do not.