Actually I predicted you'd try use this argument hence why I brought up Noctowl. It can Whirlwind and Hypnosis too you know and is faster than Walrein, therefore it doesn't necessarily have to try outstall you but its definitely disrupting your play.
<rant>
If you're going to bring something, please state it up front. This pisses me off like nothing else when you'd rather inflate your own ego instead of focus on the matter at hand. This is not a game to win/lose. The purpose of this thread is to consider various pokemon and create a stable and fun UU environment.
</rant>
Anyway, okay, so you whirlwind me out. Hypnosis is really a null argument as you can't do it while my sub is up. So at worst, you'd whirlwind me out. I'll admit this as a "disruption", similar to how roar suicune works in OU. However, it is still far from a true counter to walrein.
Why the hell would I run a Muk with absolutely no attack? Thats a massive waste if I ever seen one and 1HP damage is hardly anything when Taunt would stop you attempting any sort of stall. For example my favourite is Subpunch I could run only 180 s.def EV's with Adamant and 404HP and Walrein would NEVER break Muk's sub in one Surf and Taunt would leave you in a bad position.
Now this still leaves me with enough attack EV's for 302 attack which against even a full 252/252 +def Stallrein takes well over regions of 51-60%. That sends you onto a defensive with a locked down Pokemon and the fun part is Muk still has room to further enhance its defenses and also a slot for one more move in this kind of setup. Of course you could run a more offensive Walrein but then you compromise much of what makes the defensive set so effective.
While we're on the subject of changing sets around to suit arguments, remember, you're still dealing with a specific set of a Pokemon and I have yet to explore the possibilities.
IE: Muk may be a counter to the classic stallrein, but what if instead I was running protect/ sub / Rock Slide / Earthquake walrein? Now neither Noctowl nor Muk would like to take a hit. Or Blizzard/Earthquake walrein?
Muk is still the best of the bunch you described, taking only a 3-hit KO (potentially 2-hit KO), but Walrein certainly breaks the sub and outspeeds him.
It looks like a stallrein, it plays like a stallrein, but for sure, every single stallrein counter/disruption listed so far (Noctowl, Toxicroak and Muk) will lose out, while Walrein still is quite a general player. Rock Slide / Earthquake is a type combination with few resists, and by definition any pokemon who attempts to soak up Toxic Spikes will be at least neutral earthquake, and most likely weak to it. Those who aren't would be grass/poison (weak to blizzard) or bug/poison... and Venomoth doesn't exactly have the best defenses on either side.