The horses that play in 'War Horse'?

I just read that they used a Quarter Horse as one of the horses that played Secretariat in the movie...I wonder why they used a QH in a movie about a thoroughbred?

And I believe that in the book War Horse, the horse is an Irish Draught...where as in the movie Joey is a thoroughbred.

In the book I believe Joey is a Tb x of some sort. It's been a long time since I read it.

I just looked it up, in the book he's a thoroughbred/Irish draught cross. Very beautiful horses!