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Well, whether that's the case or not (I have no idea, as if anything I've always considered the 'golden ratio' stuff at best a very partial aspect of visual beauty), there are so many reasons why we like horses, and if we like someone we also tend to think of them as more attractive than someone might do who doesn't like them.
As for handsome, they have gorgeous eyes and many are very elegant in their movements.
Many horses have very sweet, gentle personalities. There is the aspect that we 'break in' horses to subjugate them to our will, but many people don't do that and instead just befriend them.
Do we consider horses to be so handsome because their face length / face width approaches the golden ratio?