A real snake up to the cut edit where he strikes with the knife. Fake mechanical snake after that. Couple reasons. A show as popular as this one, on a network as large as this one, does not want to run afoul of animal rights groups. And, I don't care how bad ass an actor portrays themselves to be in their roles, you are gonna' be hard pressed to find one who's gonna' straight up take out a rattle snake like that.