The reason most people regardless of info believed they wouldnt spawn is simply because they were the vanguard controllers mounts,
as such to spawn them full time the vanguards would have to be a permanent in game addition which would ruin the point of me soloing these guys ALL they time (had it to a science times, places, tactics they were easy) and collecting the shields and swords as well as taming mares for pretty much the whole guild (DC* biggest guild on gl over 270 vet members, thats alot of dreads!!!)