Some people define it differently, but for the most part they are commonly those brown spots you find on ponies. There is also pin-dot mold and some other problems that ponies tend to have. The brown spots are attracted by bacteria as are some forms of mold. I always wash my ponies with soap and water when I get them and try not to touch other things like your face etc so you dont spread any bacteria.
"When the Woodland Halls are Green and cool..
And the wind is in the West..
Come back to me.. Come back to me!
And say my land is best!"
-Treebeard, The Two Towers