The National Army Museum has an exhibition opening on 22nd October (and running until August 2012): War Horse, Fact and Fiction. It takes Michael Morpurgo's War Horse as a hook on which to hang the rest of the exhibition, which is illustrated with artefacts from the Museum's collection, "encouraging visitors to think about the millions of War Horses who have supported the British Army across time".
Entry is free.
If you've read the book, you'll know that it opens with a description of a picture hanging in a village hall, which shows Joey, as painted by the man who rode him into war. Although Iddesleigh, the village, exists, as does the village hall, the picture up until now has been a complete fiction. Equine artist Ali Bannister has now made fiction fact. Michael Morpurgo has commissioned her to do two portraits, and one will hang in the village hall. There's an example of the picture on The Times site - sadly it's behind a paywall. If I can find a copy which is a bit more accessible I will post a link.
Edited to add: the picture will be on Ali Bannister's website within the next 4 weeks, and will also be appearing at the War Horse exhibition.