While DreamWorks wouldn't comment on the story, the site has confirmed the news with one of their sources. Details weren't given about Hiddleston's character, only that he was "one of the leads" in the film.
Even though DreamWorks denied that Eddie Redmayne is playing the protagonist in the film, Albert, The Playlist, which originally reported on the Redmayne casting break, is still standing by the story, claiming that their sources are, "confident about the tip."
We'll be sure to keep you posted on any further details on War Horse as soon as more information comes in.
War Horse was released December 25th, 2011 and stars Emily Watson, Toby Kebbell, Benedict Cumberbatch, David Thewlis, Tom Hiddleston, Eddie Marsan, David Kross, Peter Mullan. The film is directed by Steven Spielberg.