The only way I was able to reliably get the Primal horse mounts (I managed to get Garuda's horse on my 399th run... Nightmare on my 400th of Garuda Ex) - Ifrit, Titan, Ramuh, Leviathan and Shiva - was forming my own party in PF with the comment "Farm pt, Horse drop = Lootmaster rule in order of join. Everything else, FFA. Multiple Runs please stay after drop". I'd make a list of the people who join, in order of when they joined and off we go. It worked very well and people in my runs always got their horse and some primal weapons. Of course it only really worked well since we have Unsynced parties for the first 5 primals obviously.

Yeah, I had some rage at me for using the Lootmaster and demand I make it FFA as other parties do, but at the end of the day they got their horses. Those who didn't like the setup, were free to leave.