Could be a mech that, for some reason or another, never sees production. Being nuked by WoBBies is a good start.
As for the Dire Woof, its been established that it takes a lot more resources and infrastructure to build an Omnimech facotry and then build Omnis then for a conventional Battlemech. Vlad was reluctant to build omni foacotries in the Inner Sphere and had decided that the clan could suffice on homeworld supplies. Following the start of the Jihad, the Wolves have been left short on supplies and tech, hence mechs like the Tundra Wolf and Sun Cobra - basically, all they are capable of producing.
Given that the Outreach factories are so much irradiated rubble, I doubt that we'll ever see Dire Woof production in the IS again.