The best you might do, in tracking down, that brand, is to hope it was registered with the state the horse was earning a living in.
Google "livestock brands in ______________ and fill in the state.
If you think the horse was branded in Florida, then start with Florida.
If the brand wasn't registered, you're up the creek in that regard
As far as what breed(s) the horse might be. That depends how far you want to pursue that and how legitimate DNA testing is.
There's a lady on another forum who got one of her horses DNA tested. She had to guess at the breeds it might be. I thought she'd made some accurate guesses but, when she posted the DNA results back, they weren't anything near what I thought that horse resembled.
Meaning, I don't know how much stock to put in DNA testing when it comes to learning what breeds might have gone into a horse.
If you're happy with the horse and the horse seems to be healthy, while it's always good to know history, sometimes it's the better part of valor to not open up Pandora's Box