Is chat familiar with Madeleine L'Engle,
A Swiftly Tilting Planet or other books from that series? Can they match
scifi.stackexchange.com/q/251810/4918 Sci-fi novel involving an animal that could be ridden like a horse and used to travel through time ? I never read those books, but seen its cover illus shows someone riding a steed on it, and it has time travel.