My 6 year old granddaughter wanted The Black Stallion to read.  I ordered a copy here from PBS, but the book I received was not for a 6 year old.  More for 12-up.  So, does anyone know of the book that is for younger readers?  .  

You might try Back Beauty, that's a classic too. I know there are several versions written at different reading levels. And I am sure there are quite a few children's books written about horses. Do a genre search for children's fiction and sub category 6-9? with a keyword of horse and you'll find what's available. It's possible there is a version of Black Stallion rewritten for younger readers as well.

Yes, I searched under the children's age level.  I remember years ago a younger age book was available, but just can't find it.  Even doing an amazon search.