Imprisonment leads people to desperate places. This is how the tale of Clan Kreepus begins. Life in Gnaw Pit was okay for Kreepus, but Clan Grikk came along and slavery seemed unavoidable. Thankfully, Clan Eshin had a solution- in exchange for servitude, Kreepus would be free with help courtesy of Eshin. Service seemed preferable to slavery, so the enslaved agreed. A shipment of poisoned daggers made it into Gnaw Pit, but the presence of handcuffs made using daggers in the conventional sense impossible... thankfully, they created a more creative solution- use their tails instead. Their victory was immediate the following morning, and the union (and mimicry) with/(of) Clan Eshin has held firm ever since.
Interesting fact: Clan Kreepus members almost never show their faces! Always hiding behind masks, cloths and (in desperate cases) old rags, nobody ever sees a Kreepus Skaven's face... and lives to tell of it.