poltroon, I wrote a book report on For Love of a Horse in fourth grade, and my teacher kept correcting “Arab” to “Arabian.” Technically, I suppose she was not incorrect, but I was like, “It’s in the book! That’s how it was written!”
Although I’m a guy, I was about her age when I read the Black Stallion series…still remember it as one of my favorite series today. She needs to check that out if she hasn’t.
When I finished those, I moved on to Misty of Chincoteague series. They were also pretty good but as a young boy, I couldn’t help but think they were more geared for girls so I didn’t enjoy them as thoroughly - might be perfect in your case.
Not sure how avalible these are anymore, but I loved the Galloping Detective series as a kid. Triple Threat is the first of six.
The “Bonnie Books”, starting with The Sweet Running Filly. The were favorites of mine as a kid, and they’ve recently been re-released.