"War Horse" is based on the novel of the same name by British author Michael Morpurgo. While the book is set against the backdrop of World War I, the characters and stories depicted in the book are fictional. However, the plot of "War Horse" was, indeed, inspired by a real encounter. 

In 1976, Michael Morpurgo and his wife founded Farms for City Children, a charity that gives kids the opportunity to immerse themselves in rural life (via Farms for City Children). During one of these excursions, Morpurgo met a boy named Billy who struggled with stuttering. Morpurgo later found Billy in the stables chatting away with a horse. The boy's stutter had disappeared. The author described the magical moment to The New York Times: "All the fear had gone, and there was something about the intimacy of this relationship, the trust building up between boy and horse, that I found enormously moving, and I thought: Well yes, you could write a story about the First World War through the eyes of a horse, and yes, the horse didn't understand every word, but she knew it was important for her to stand there and be there for this child."

Morpurgo was further inspired to write "War Horse" after encountering World War I veterans who discussed their experiences with him. These men are thanked in the book's dedication. 

Despite the film's incredible success, Steven Spielberg was actually not the first to adapt "War Horse."