Horse manure is great for dung loving varieties such as Agaricus, Panaeolus & cubensis. It's dry and easy to work with, high moisture holding capacity and hydrates easily & and is nice and fluffy... Mmm, now doesn't that sound like something a mushroom would love. We find you antibiotic free h-poo, mill it and send it to your door.
Breaking down lignan is a difficult job, in fact only fungi can. Horses only partially digest the various grasses they chew on. This is great for secondary decomposers such as mushrooms.