Feral Orks
Everything starts somewhere, and many Ork Waaaghs start here. Orks, being Fungi, reproduce by periodically releasing spores from their skin, and eject a mass-release upon death, so it stands to reason that old battlefields'll be saturated in them. Once the original inhabitants of a planet've all been killed and the Orks've got bored and left for places and wars new, these spores'll still be there, and under the right conditions can germinate and grow, restarting a decidely more-Orky ecosystem than the one they supplanted. Of course, cut off from pre-existing Ork "civilisations", these Feral Orks'll have to start again from scratch, figuring out such rudimentary concepts as language and technology for themselves.
Feral Orks enter the world firmly in the middle of the food chain, preying upon smaller beasts, and being preyed upon by larger ones, in a constant free-for-all where only the strong'll live to see another day. As more Orks sprout, they form into loose tribal units, and individuals begin to develop specialisations to better support the community, such as Pigdoks and Boiler Boys. These disparate Tribes compete with local predators and each other for resources, until a particularly brutal or cunning Ork emerges who can unite them all under his banner, becoming their first Warboss.
Now with the advantage of numbers on their side, the Feral Orks can push the predators back, and start carving out a territory of their own. A Tribe'll often name themselves after the local predator that troubled them the most, making it a central part of their identity. Sallying out into undiscovered country, they start rediscovering the "relics" of their progenitors, the ruins and weapons of the invading Orks that first came here. Whilst much of it's of unclear purpose and obviously the product of a more advanced culture, eventually they discover that stuffing the metal drum with smaller metal darts and pulling the finger-lever makes it go boom.
As a Feral Ork Tribe progresses through the various Bronze, and Iron ages, eventually they gain enough knowledge to reverse engineer technology of their own, and they can finally build Starships that let them rejoin their brothers in space.
On the Tabletop
The Feral Orks received a full 3rd Edition army list in a Chapter Approved column of White Dwarf issue 260, summarised here. They could take Warbosses, Nobz, Pigdoks, and Wyrdboyz as Headquarters, Brutes, Trappas, and Stikk Bommas as Elites, Huntas, Wildboyz, Gretchin, and Madboyz as Troops, Boarboyz, Herdas & Squighounds, and Junkas & Junkatrukks for Fast Attacks, and Squiggoths and Lobba Batteries as Heavy Support.
8th Edition
With Psychic Awakening giving Orks Specialist Mobs which can be chosen instead of the traditional Clan Kultures, Feral Orks're back on the tabletop once more, though they've apparently been split up into three separate groups. First're HUNTAS, which give all ORK INFANTRY fully within terrain a 5++ Invulnerable Save and +1 melee AP thanks to their Sneaky Devils Special Rule. Then you have FERAL ORKS proper, which assigns the Wildboyz Special Rule to WARBOSSES, WEIRDBOYZ, non-BIKER or MEGA ARMOUR NOBZ, and BOYZ, giving them a 6" Pile In move and letting them roll 3d6 and pick the highest when Advancing. And finally there's MADBOYZ, where you roll 1d3 at the beginning of each Battle Round, and all ORK INFANTRY and BIKERS get one of three buffs for its duration, either +1 to Armour Saves, Fearless, or +1 Strength.