I thought that as interesting to had to see what was on good old Google!!!
There were always people when I was growing up that could have been described as "horse whisperers' because they had some inner quality that seemed to enable them to calm a horse down where others failed - a cousin of my grandfathers that I spent a lot of time with used to take 'impossible' horses to break or sort out and I watched him get them relaxed and settled really fast with no running them round or stuff like that. I know that foxhunter also has memories of someone from her youth that was the same
I did find this which is interesting in itself but how factual it is I don't know
Secrets of The Horse Whisperers
Anyway, surely Xenophon is the first "horse whisperer," or at least the first one to write his methods down. And he was a Greek.