A YA book like Beautiful Joe but is the story of an abused horse. The horse’s name may be Charlie. Looked up Five o’ clock Charlie but it doesn’t seem right, seems too happy (is there anything about him being abused by a first owner?). I can’t give any more detail as I never read it. I want to buy it as a present for a friend who read it as a child, he’s in his mid 40s.
It’s definitely not Five o’Clock Charlie, who isn’t abused. It might be Old Charlie, by Clyde Robert Bulla, about boys who rescue their old camp horse. Here’s a link to the Kirkus review of the book
https://www.kirkusreviews.com/book-reviews/clyde-robert-bulla-14/old-charlie/
The most obvious book would be ‘Black Beauty’, but assume that this would have already been checked. Might it possibly be one of Jenny Oldfield’s ‘Hurricane Ranch’ series: perhaps ‘Steamboat Charlie’, where the horse *is* called Charlie? Another possibility might be Clyde Bulla’s ‘Old Charlie’.
I found this book, but it was written in 1994; so it doesn’t seem old enough.
https://www.goodreads.com/book/show/1985404.A_Horse_by_Any_Other_Name
There is a book titled “Horse Haven” by Nancy Caffrey, involving children’s attempts to doctor a horse. At one point a human gives the horse medicine which the horse spits into the human’s ear. The book is older, rare, and expensive as all get-out.