Ask yourself which animals (if any) have self-awareness and you'll open the door to an age-old mystery. A self-aware person is conscious of their own thoughts and feelings, and maybe flaws. In the animal world, the question tends to be more along the lines of "Is this creature aware of itself as an individual separate from the environment?"
One possible tool for answering the question is called the "mark test." A mirror is introduced into the creature's surroundings. Most animals initially treat their reflections like other animals, and respond socially; the mirror test involves a cool-down time, waiting until this is no longer the case. At that point, researchers apply a visible but harmless mark to the animal in question, usually on the face or the ear but always in a location that the animal can't see without use of a reflection. The thinking goes that if the animal notices the mark in the mirror and can connect this sight to their own face or ear--for example, trying to remove their mark--then the creature must be capable of at least some degree of self-awareness.
(There is some controversy attached to the "mark test." It may produce false negatives. For instance, non-Western children sometimes "fail" the test at even six years old, an age by which we can all probably agree there is a distinct sense of self. Individuals watching their reflection can display other behaviors which suggests they know what they're looking at; for instance, some elephants who failed the "mark test" stood in front of the mirror making repetitive motions, as if testing to see that the reflection would follow suit. The test also does assume that everyone would be captivated by seeing a mark on their bodies, which may simply not be the case.)
Scientists have attempted this test many times, with a wide variety of animals. Few pass. Of the animals who do succeed, they tend to belong to species known for their intelligence: chimpanzees, some dolphins, two magpies, and a single elephant. Now, researchers in Italy report a new unlikely winner: horses.
The researchers placed a mirror in a horse training area. Once the social response among the horses had passed, the researchers continued. In the first stage, they applied a cross-shaped blob of colorless ultrasound gel on the horses' cheeks. In the second stage, they used an ultrasound gel with some pigment to it, resulting in a visible mark. The researchers figured that if the horses paid more attention to the mark they could see in the mirror, it would imply self-cognition.
And indeed, the horses spent on average five more minutes scratching their faces in the mirror when the mark was visible. Of the eleven horses who participated in the study, ten of the horses spent longer on the visible mark than the clear one. This implies that most of the horses could, in fact, make the connection between their reflections and themselves.
What we saw in Italy was only a single study, and it is too early to say that all horses are self-aware. More studies would be needed. Still, this is one exciting piece of research that implies horses may be smarter than we've been giving them credit for.