Black Horses for the King

Galwyn, son of a Roman Celt, escapes from his tyrannical uncle and joins Lord Artos, later know as King Arthur, using his talent with languages and way with horses to help secure and care for the Libyan horses that Artos hopes to use in battle against the Saxons.

The publishers describes this as McCaffrey's "first historical novel for young readers", and it is probably not a genre book, in that there appears to be nothing actually fantastic about the story.
"This novel was developed from the author's story "Black Horses for a King," which first appeared in the anthology Camelot, edited by Jane Yolen and published by Philomel in 1995." In her introduction, McCaffrey credits Yolen with "all these 'minor' changes that became major improvements in my yarn."