Dear Appy,
Why does my dog eat horse poop?
Signed, Concerned Dog Mom

Dear Concerned Dog Mom,
Ew. Why do dogs do anything? Some of them eat their OWN poop (known as coprophagia for the uninitiated.) The human at my farm has a dog that eats walls. Figure that one out. But seriously, most humans with horses agree the smell of horse manure is pleasing and given the amount of time they spend moving it around, it’s obvious that manure is a high value item. Dogs love to please humans by loving the things their humans love. Ergo, dogs eat manure. And roll in it. 
Sincerely, Appy