A quote that I though appropriate to send us into 2010...
Most horses pass from one human to another - some horsemen and women are patient and forgiving, others are rigourous and demanding, others are cruel, others are ignorant. Horses have to learn how to, at the minimum, walk, trot, canter, gallop, go on trails and maybe jump, to be treated by the vet, with sense and good manners. Talented Thoroughbreds must learn how to win races, and if they can't do that, they must learn how to negotiate courses and jump over strange obstables without touching them, or do complicated dancelike movements or control cattle or accommodate severely handicapped children and adults in therapy stables.
Many horses learn all of these things in the course of a single lifetime. Besides this, they learn to understand and fit into the successive social systems of other horses they meet along the way. A horses's life is rather like twenty years in foster care, in and out of prison, while at the same time changing schools over and over and discovering that not only do the other students already have their own social groups, but that what you learned at the old school hasn't much application at the new one.
We do not require as much of any other species, including humans. That horses frequently excel, that they exceed the expectations of their owners and trainers in such circumstances, is as much a testament to their intelligence and adaptability as to their relationship skills or their natural generosity or their inborn nature.
That they sometimes manifest the same symptoms as Romanian orphans - distress,strange behaviors, anger, fear - is less surprising than that they usually don't.
No one expects a child, or even a dog to develop its intellectual capacities living in a box 23 hours a day and then doing controlled exercises the remaining one. Mammal minds develop through social interaction and stimulation. A horse that seems stupid might just have not gotten the chance to learn!