The RINO-in-Chief

We keep hearing from the Left/Democrat Mueller Apologists that Mueller, and some others that have big question marks against their names, is not a bad man, that he is actually a Republican. What? Well, it's what Wikipedia says, anyway. Mueller is a Republican... However, I also found RINO on Wikipedia: I consider it a great oversight on the part of Wikipedia that Mueller's name was not on the RINO entry. My father taught me a basic truth when I was young. He asked me how many legs a cow has and I said, of course, "Four." He then asked me if I called the tail a leg, how many did it have and I said, "Five." He said, "Nope, just because you call the tail a leg does not make it a leg. The cow still has four legs." He went on to say that just because someone calls a thing one thing or another does not make it true. Just because they call Mueller a Republican does not make it so. He's either a Democrat in Stealth Mode OR a RINO. But what is the real difference between the two?