While the idea that 12,000 North Korean troops will replace "massive Russian losses" (obviously proving that, in that case, the losses aren't really massive) is ridiculous, the possibility of Pyongyang sending special forces shouldn't be ruled out entirely. North Korea has a sizeable SpecOps Corps, but lacks combat experience. Thus, it's not outside of the realm of possibility that Pyongyang itself asked Moscow to send some of them to Ukraine or at least Russia, where they could train with Russian special forces.