I've just done a little search for Steve McQueen films, and came across The Reivers, a 1969 film, which makes mention of a racehorse! Not sure if that's the one you're after, but you could go to this link and read the plot synopsis to see.
http://www.allmovie.com/cg/avg.dll?p=avg&sql=1:408
57~C
Hope it works! (I think the symbol before the C might be a bit dodgy!) If it doesn't, just type 'steve mcqueen the reivers' into Google and you'll have plenty of sites to choose from!