This has now been patched by Rockstar.
Click on this link to see my video on How to get the Silver Dapple Pinto at Face Rock.    • HOW TO GET A SILVER DAPPLE PINTO at F...  
It is an easy and sometimes quick method to get your pinto.

Before the patch:
The Silver Dapple Pinto Missouri Fox Trotter is one of the rarest and best horses in the game. It usually is available only in the later chapters with John Marston. However there is a way to Arthur can get it in chapter 2. The method relies on a random spawn and patience is usually needed but, if you are lucky, it may not take long. I was lucky in this video and found it almost immediately.

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