This video from Horse Racing Ireland gives an insight into what makes a winning racehorse:

And this article from Love the Races shows what leading female jockey Hayley Turner personally looks for in a horse:

http://www.lovetheraces.com/new-to-racing-/what-to-look-for-in-a-superstar-racehorse/

1. The body "I like to see a good broad chest, and with a horse’s back end you want to see the muscle. If you are looking at two-year-olds that haven’t fully matured, their hind quarters are always higher than at the front end. That is a sign of weakness. They will level out in time.” 

2. A Gleaming Coat "This is an important one. If they’ve got the shine – a good skin, a good coat – that’s the health inside them coming out. You wouldn’t want them to be too woolly at this time of the year.” 

3. Sweating "It’s not unusual if a horse is sweating on its neck, especially on a warm day. But if they’ve got sweat dripping off them, you wouldn’t want that – boiling over before a race. That’s a sign the horse is overwhelmed by the whole atmosphere." 

4. The Head "The size of a head is only relevant if they’rein a photo-finish! The eyes want to be alert.Big ears are a sign of a genuine horse, and big nostrils are always a good thing (to get lots of air into the lungs).”