Some of those drugs are legal but have to be administered a certain number of days before a race.
"The withdrawal guidelines published by the Kentucky Horse Racing Commission recommend that the medication not be given within 14 days of a race. In this instance, as an additional layer of protection, Gamine's veterinarian last treated her with betamethasone 18 days before the Oaks."
I wonder if the horse got the shots Legally, earlier, but his system didn’t clear the metabolites of the drug within the expected withdrawal time.
We had to watch for this when we were milking cows to avoid putting “hot” milk in the tank. Some of the cows took a lot longer than expected to test OK and clear of medication. If we were responsible for a condemned tanker load, we paid for it.
The horse might have been in the same situation.