In the writings of the infamous warlock Gul'dan, he mentions Warlock clans that are apparently at least partially separate from the Shadow Council. Orgrim Doomhammer slaughtered them, and Gul'dan replaced them with the death knights.
Mentions of "Warlocks" throughout these sources may or may not refer to the clan(s).
The "Warlock clans" may be euphemisms for more formally named clans, such as the Shadowmoon clan and Stormreaver clan, who might have included warlocks in their ranks. Perhaps they refer to the warlock class as a group, not unique to any clan. It implied 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. In Warcraft II: Tides of Darkness, all these sources are mentioned separately. Their interchangeability is unknown.
Similar confusion surrounds the rarely-mentioned Necrolyte clan.