I think you are absolutely giving these guys too much credit
I talk to several other vets that work in various states, and everyone knows what the current hot fad drugs are these morons are trying.
These guys are not chemists, nor pharmacologists, nor magicians. They are no more sophisticated chemically than the average meth dealer. They are not smart. They are not "one step ahead" all the time.
Believe me, if they were that adept, they'd easily make big bucks doing it legally
They buy regular human drugs and other made up crap off the internet, or get stuff through other grey-market sources, and throw it on the wall (into the horse) and see what sticks. "Let's try Viagra to see if the cardiovascular side effects will make the horse run faster"
Geesh, the harness guys try stuff way before the TB guys do
The TB industry knows what's coming it's way.
There are not hundreds of magical, undetectable moveup drugs that make the horse run faster and farther, without detectable side effects or obvious changes in the horses' physiology (pupil dilation, heart rate, respiratory rate, muscle activity, sweating, etc)
I'm not saying there are not guys that cheat, or that there is stuff being used that's currently undetectable - of course there is.
It's just not the magical mystery tour chemical free-for-all some think it is. C'mon guys, we have to base it in reality.