Blackhand, chieftain of the young Blackrock clan and a Raider in the Sythegore Arm, was well honored by most Orcs within the Horde. This is the only mention of the term. It may or may not apply to all orcish raiders.
Orgrim Doomhammer disbanded all raiders within the Horde after he took power because he feared that they would betray him in an attempt to put Rend or Maim on the throne (which was untrue; it was an attempt by Gul'dan to gain the Warchief's trust after his near-brush with death). By the beginning of the Second War, wolf-riders were no longer common within the ranks of the Horde's armies. Therefore, whatever the Sythegore Arm was, it likely stopped existing.