It has been a while.
I want to discuss some things about horses that I have observed over a few years. The setting of these "incidents" is within the confines of a farm, even to a paddock. It involves only two horses so far, one of which I owned and the other I saw on a video. The behaviour was so remarkably similar that I have thought about it for a while to come to the basic conclusion that I have.
The "incidents" involve two stallions, one actually a three year old colt, but the other is about eight or nine years old. Both animals were raised very close to humans and have lived as domestic animals all their lives. The older stallion is a breeding animal, the colt was old enough to breed but not the opportunity as he was isolated with just his dam as company.
Both "incidents" involve the motion of rearing, standing on the back legs and walking a few steps. These actions were done voluntarily and in the presence of humans. The stallion was playing around with a human in his yard and involved cantering around his paddock, then going to the human and rearing up next to him. There was no threat in the stallion to the human. The colt was doing the rearing and walking while holding a feed bucket and throwing it around.
The colt would take the bucket by its handles in his mouth, as the handles were the easiest way to hold the bucket. He would rear up and take five or six steps forward. Because of my close interaction with this colt, I really felt that he was imitating the upright walking of humans. When I saw the stallion doing the exact same thing in the presence of his human, I realised he was doing it as well. Showing the human he could walk upright as well.
It may not seem unusual at first that a horse would behave in this way, considering that both animals have spent all their lives with humans. When you think about it deeper, you would realise that it is indeed unusual for an animal to think like this. First of all the action of rearing is not an easy one for a horse. It has a rigid spine (unlike a flexible spine of, say, a cat) and is a grazing animal. Rearing is often seen in young horses as part of their play, both colts and fillies do it. Mares rarely rear once they are adults, although they are still capable of doing it. Well fed domestic mares will rear in play sometimes, but not often. Mares are quieter in that they think mostly of either feeding a foal or just wandering looking for grazing. A stallion in the "wild" will rear as a threatening behaviour towards other stallions/horses. Rearing is used by stallions in fighting as a position of height can allow the stallion to gain an upperhand in a fight and defeat a challenger. These are the main natural uses of rearing in horses. Rearing for no apparent reason other than play just is not usual.
For these two stallions to rear up and actually walk on their rear legs only is very unusual and indicates to me that there is a deal of thought going on with the animals. The imitation of humans this way is unusual. Very few animals other than primates are capable of imitation of humans.
As to why I concluded that this was actually what the two stallions were doing, I must admit I cannot be certain, other than the way the walking was done. And the need for approval of both horses afterwards. My own colt came up to me afterwards and was cocky and acting in his way of telling me he was clever. It is a body language he developed for me. Body language among horses is important as it is their main means of communication.
The other stallion was beside his human when he performed the "incident" and afterwards made some cocky behaviours similar to my colt and then repeated the whole thing. It was as though both stallions needed to know they had been seen doing what they had done and afterwards wanted to know if the act was good to the human opinion.
Thinking and the ability to think is something humans believe is solely their domain. Horses were domesticated as beasts of burden and for transport, particularly during military conflict and speed. An ability to think/reason is not required on the part of the horse. However, both of these stallion do not do alot all day. They have time to think. My colt worked out that he could stand behind a shed when it was feeding time and jump out at me to give me a fright (from not expecting him to do this). He never resorted to biting or kicking when he did this, just jumped out, waited for my reaction and then trotted away, as though he was laughing.
The colt was a registered Thoroughbred, and the stallion is a registered Arabian. Both breeds are reputedly "hot blooded", and the dam of my colt is spooky even though we have tried many things to de-spook her. The colt has since died, although the stallion is still alive.
If this incident had just been my colt doing it, I would perhaps have thought I was a bit one eyed in thinking him smarter than he really was. Having seen another horse act in the same way, when the two animals had never met in any way, shape or form nor was either stallion related to each other in any way. I have never had personal interaction with the stallion. I saw the stallion on a video that appeared to have been filmed from a mobile phone.