With enough training, Proops thinks horses could ace all of the tests, but dogs don't seem to need this extra tutoring.
Surprisingly, our closest living relatives, chimpanzees, also sometimes do poorly on related tests, yet it's suspected they might have not been motivated enough to pay sufficient attention.
"Primates that have had extensive experience with humans tend to perform as well as dogs (on such tests)," said Proops.
The researchers theorize that the way we've domesticated horses has affected, in part, how they respond to us now.
In the past, horses were mostly valued for their size, beauty, strength and other qualities not always associated with behavior. Dogs, while bred for these traits too, were also selected for companionship characteristics.
Depending on how horses are domesticated and trained in future, they may have the potential to catch up with dogs as being man's best understanding friend.
A recent study conducted by Carol Sankey of the University of Rennes, for example, determined that horses recall positive interactions with individuals, even when the horse and human are separated for months at a time.