The CS montior has a nice piece about the social intelligence of dogs as studied in the laboratory of Eotvos Lorand. Dogs are incredibly attuned to humans and in particular their owners. Lorand's group has been able to show that they consistently do better than wolves, and even chimpanzees, in tasks requiring rapport with the human minders.
The most interesting part to me is the implication that ability to interpret social cues is its own kind of skill, which may be very different from intelligence per se. The difference between dogs and wolves also suggest that this separate skill has been actively bred for in domesticated dogs.
I'd be very curious how horses come in in this sort of test.