This article is from 2015 – more animals have made the list since, including mice and fish in 2023.

It’s worth noting there are major flaws with that test, including that it’s known not to work on animals for whom sight isn’t their primary sense, and the animal’s socialisation or lack thereof may play a part. In the case of mice, it only worked in certain circumstances:

However, the mice only showed this self-recognition-like behavior if they were already accustomed to mirrors, if they had socialized with other mice who looked like them, and if the ink spot was relatively large.

The study on mice included more than simple observation, too, which is very interesting:

The team identified a subset of neurons in the hippocampus that are involved in developing and storing this visual self-image, providing a first glimpse of the neural mechanisms behind self-recognition, something that was previously a black box in neurobehavioral research.

Using gene expression mapping, the researchers identified a subset of neurons in the ventral hippocampus that were activated when the mice "recognized" themselves in the mirror. When the researchers selectively rendered these neurons non-functional, the mice no longer displayed the mirror-and-ink-induced grooming behavior. A subset of these self-responding neurons also became activated when the mice observed other mice of the same strain (and therefore similar physical appearance and fur color), but not when they observed a different strain of mouse that had white fur.

Dogs don’t pass the traditional mirror test, but they do pass a scent-based equivalent.

It’s an interesting test, but an animal failing it may be down to inherent flaws in the test rather than a lack of self-awareness or cognition.

e: missed a few words