Some horses are fed with something that makes sh*t out of their poop. I have no problem with the manure smell (I pick mine every day), but piles from some horses smell awful (on parking lots or on trail).
I also love the smell of horse, especially my horse.. he smells different. And a nice different. I also like the smell of hay and manure.
I hate the smell of a barn that has poor grooming habits (and it's not their fault of course) but we visited an Andalusian breeding and training for high levels barn. Holy cheese it reeked, my nose burned so much. Usually when I smell horse urine (in the arena..) it smells like apple cider.. but this was terrible. It was like all the yuck was fermenting.. so gross.
Smelling their necks.
LOVE the smell of leather, and I love to clean and oil my leather tack
Thrush or old smegma. Yuch!!!!
Really dirty stall.
Hate to mention this one, and hope you never have to smell it--when your old horse (one of mine lived to be 35 yo) passes away, and you end up waiting a few days for Animal By-Products to collect the body--AFTER I've removed the mane and tail parts for saving and future sobbing!!!--the smell can only be cleaned up with bleach.