Or to keep it a bit more scientific - sheep and horses, unlike cows, do not have extra stomachs full of specialist bacteria which produce enzymes which break down cellulose into smaller digestible molecules. This means that in horses and sheep (and various other animals), fibre is not digested the way it is in cows. As a result it passes through, which explains the composition of horse manure, which is not that different from the original grass.
It also means that horses and sheep need to eat a very large quantity of grass for the small digestible percentage to add up to enough useable food to sustain them. Luckily sheep have all day to graze, so do quite well on grass. They also do quite nicely in a casserole...
Quack!
GtD.