Throat infections. Not visible to eye or feel, only on the racetrack. Only a scope will tell you prior to racing them, always worthwhile to get them done because if you do race with one it will set you back a month.
Sore shins. All horses get them at some point or another. 3 months in the field and they never recur, some trainers ice them up and race them (they think that if they cant feel them they arent there). Bad idea, if the horse makes it to the end of the season it will break down during the next.
Ringworm. Lads in racing yards are too braindead to comprehand 'seperate this, seperate that' so if one gets it the whole yard will.
Low grade viruses/anaemia. Only a blood will show either for definate, always good to check before a run due to reasons stated for throat infections.
But you also have to realise these horses health are monitored so closely - at the big barns the horses are scoped after every workout, so they are being scoped every week to 10 days, so things get noticed. Penelope The Pet Pony has things go wrong with her too, but her owners are never aware.