Here in the USA many Thoroughbreds are beginning broke as late yearlings and are racing as late 2 year olds.
They are working, conditioning and learning to run at the home farm and shipped to the track for serious works done.
I don't know the rules of Thoroughbred racing in AU but here in the USA if a horse steps foot on a racecourse property they must be tattooed, lip tattooed
which then takes much of the guess work out of age and real name, racing career or not..
I think it is very similar elsewhere but could be wrong.
Many horses are "duds" as far as racers go...no desire to run like they must, a conformation fault that now compromises their soundness and speed ability, a physical issue...most just are not fast enough nor have the temperament to race and win.
Racehorses here don't
get branded, not like that on their body.
Your horse could of worked on the track as a track-pony, very possible.
Played polo, why not. To my understanding any breed can be a polo pony...you again just need the speed, agility and want to make it happen.
The brands though make me think more Quarter Horse than Thoroughbred.
The only other horses I can ever remember seeing with multiple brands, multiple locations were either warm-bloods who went through that grading/certifying process identifying stud and mare and think it is Lipizzaner is tattooed like that on shoulder, flank and someplace I remember something about males being kept intact were somehow marked on their spine/upper back???
Don't quote me on the Lipizzaner thing and that may have been country specific where that was done...
Best of luck uncovering a past history...
What I would do is contact the person you bought the horse from and ask those questions..
If she came from a racing farm/stable, the guy you said trained, the papers may have been destroyed so no trace of the animal, but the animal lives today.
It is a cloudy/murky area of how many horses were racers and left the track with no history, no papers and what was known about them gone in order to save a animal from a dire fate..
You have nothing to lose by asking, some to gain but the horse is yours to have and enjoy...sometimes not knowing their past is better too. Honest.