I’m not explaining it very well, then. Let me go at it from this angle, although I don’t have citations for what I recall having read about this: That a number of previous intelligence tests on horses have indicated they’re stupid, but that the parameters of such tests were better suited to a different species which would have the behaviors and motivations necessary to do well at solving the particular problem.
Hence the chimps versus horses on object-piling tests observation. The chimps would do well on such a test, in part because, yes, they are smarter than horses, but also in part because stacking one object on another is well within the range of normal behaviors for that species. Horses don’t do that sort of thing (for one thing, they have hooves, not hands). They haven’t a basis of usual behaviors from which to extrapolate the required action.
Rephrased: You can’t give the same test to horses, dogs and chickens and expect it to tell you anything reliable about each species’ intelligence as compared to the other two.
