Found: Book from the 70s or 80s

The book I'm looking for was about a girl who gets a horse. All I can remember is that at one point the horse eats too many apples and gets colic. The horse was very sick but does survive. Any help would be greatly appreciated.

>1 LadyOtterton: Do you remember how old you were when you read it? Was the story set in the United States? Any particular state mentioned? The more you can remember, the easier it is to find.

>2 MsMixte: It was set in the United States. I was probably in middle school when I read it so I was 10 - 13 years old which would have been in the early 80s. There is another book I read at the same time that was published in 1975 so that's why I'm so unsure if the date to search would be the 70s or the 80s.

Mokey gets colic from eating green apples in Summer Pony by Jean Slaughter Doty. 1973.

>4 MissSquish: Omg! I'm pretty sure this is it! Thank you so much. I can't tell you how much this means to me! 💕

>4 MissSquish: It definitely is this book. I found it on ebay and ordered it. You have made me so very happy!

>7 LadyOtterton: There is a sequel! Winter Pony

I knew this too, but MissSquish beat me to it as usual. Doty's other girl-and-pony books are good too.