Warlock Clan

From WoWWiki

The writings of the infamous warlock, Gul'dan mention disagreements in the time of the early Horde between a Necromancer Clan and a Warlock Clan.

These may be euphamisms for more formally named clans (such as the Shadowmoon) who might have included necromancers or warlocks in their ranks, or perhaps they refer to warlock and necromancer classes not unique to any clan. It is even possible that they were in fact early clans in the Horde, which may have later evolved into the Shadow Council, the Stormreaver Clan or other groups loyal to Gul'dan.

See: Warlock (lore)

Similar confusion surrounds the rarely-mentioned Necrolyte Clan.