Dread War horse is the mount of the vanguard controllers. Kill or dismount the vanguard to tame the horse.
It's a black horse stronger than a nightmare taking 3 control slots. When wild it has a special poison area attack, but when tamed this becomes just a black cloud emitted when the owner says 'trick'.
Taming/lore requirements are, as far as has been determined, the same as a fire steed. ie 106 tame minimum to own.
Hunters Moon collated some figures on them, here's a cut/paste:
............ Max......Min
hit..........650.. ......550
sta........125.. ..............185
man.......160... ............100
str.........555. ..............500
dex........125.. ..............185
int.........160. ..............100
phy........75..............65
fire.........40..............20
cold........40..............20
pois........60..............50
ener........50..............40