This section is from the book "Our Edible Toadstools and Mushrooms and How to Distinguish Them", by W. Hamilton Gibson. Also available from Amazon: Our Edible Toadstools And Mushrooms And How To Distinguish Them.
The application of the title "horse-mushroom" to this last-mentioned species was generally supposed to be referable to the same popular traditions of which we see the analogies in the names horse-weed, horse-nettle, horse-balm, horseradish among the herbs - the prefix "horse" referring to the element of coarseness or rank growth. But in the instance of the mushroom it bears a deeper significance, as this ample cosmopolitan variety of the Campestris, which follows the horse all over the world, from stable and through lane to pasture, and which can only be grown in the manure of this animal, is now generally believed to be a secondary, exaggerated form consequent upon the following conditions:
The spores of the Campestris are shed in myriads in the pastures. The grazing horse no doubt swallows thousands of them, which, upon their return to the soil under especially favorable conditions for growth, vegetate into mycelium, and at length fructify in the full-formed mushroom. The dense white spawn of this species may often be seen beneath the manure in pastures where no sign of the mushroom itself is yet apparent.
During the writing of the present pages I have received from Arizona a letter accompanied with a sketch of a most astonishing mushroom, which my correspondent finds plentifully prevalent in his vicinity, growing in arid sand, even in an exceptionally dry season. He claims that "it is deliciously edible," and he has partaken of it several times. His sketch and description call to mind no existing form of mushroom known to me, though from one peculiarity in particular - namely, its frequently enormous size, "occasionally ten inches in diameter" - one would naturally expect to find it at least notorious, if not famous.