Smell
“The only thing he missed—besides Hemming—was Wyoming itself, its extravagant flatness, its trees so deeply green they looked blue, the sugar-and-turd apple-and-peat smell of a horse after it had been rubbed down for the night.”]]]]

Turd: a piece of the solid brown waste material you pass from your body.
Peat: a black substance formed from decaying plants under the surface of the ground in some areas, which can be burned as a fuel, or mixed with soil to help plants grow well.

Here it may refers to the smell of excrement of horse, let assumed it, why it would related to sugar, apple.
Question: what the meaning of 'sugar-turd Apple-and-peat" and "rubbed down"

Question

Smell
“The only thing he missed—besides Hemming—was Wyoming itself, its extravagant flatness, its trees so deeply green they looked blue, the sugar-and-turd apple-and-peat smell of a horse after it had been rubbed down for the night.”]]]]

Turd: a piece of the solid brown waste material you pass from your body.
Peat: a black substance formed from decaying plants under the surface of the ground in some areas, which can be burned as a fuel, or mixed with soil to help plants grow well.
 
Here it may refers to the smell of excrement of horse, let assumed it, why it would related to sugar, apple.
Question: what the meaning of 'sugar-turd Apple-and-peat" and "rubbed down"

Answer

The author is saying that the odor of the horse reminds him of the combination of these many odors mixed together. I believe that the term "turd apple" refers to a fruit that resembles an apple but is not good for anything similar to a thorn apple. When he speaks of the horse being "rubbed down" he is referring to the horse being brushed before it is left to sleep. This is done to remove debris and sweat from the horse to keep it healthy.

Answer

There were presumably notes (as wine tasters would say!) of apples, peat, sugar (sweetness) and turd lingering from the horse after it had been brushed down for the night.