And now back to your regularly scheduled abomination.
Alphas are the stuff of nightmares. That sort of terminology does get thrown about a lot for the higher level creatures, and not without reason. Certainly the carnage wreaked by Betas is easily noticed, as is the whole swaths of destruction left by a Kappa. The terror of an Alpha is subtly. Its very easy to mistake the kills inflicted by one of these undead serial killers for the work of another human being. Quite likely, their kills are under-reported for this exact reason.
Other types of reanimates have hardiness or speed on their side, Alphas not so much. They're almost as easy to disable as a normal human - but key word is disable. Like the rest of the reanimates, you're still not going to destroy them like that. You still need completely remove and then desecrate the head at some point to ensure nothing recovers the body and brings it back.
Finding it to do that though, is the catch. Chances are, it will find you - while you sleep. Alphas know how to hide, to track, to wait for targets to be distracted by other threats, and how to use tools. They can and will cut through barbed wire, scale a fire escape, enter the door, quietly sneak up on people, bash their heads in with an ax, then make an exit. Thats right - these things know how to sneak and observe.
Hard as it is to tell a type one from a human, an Alpha is even harder. They know the value of camouflage and bathing occasionally - though they don't seem to be too picky about cleanliness. Alphas move as fluidly as a normal person, not with the stiffness of a One, or the loping of a Two. The lack of speech is similar to other reanimates, as is the high body temperature, but those signs are not obvious at first glance.
Some say you can tell them apart by their curiosity - Alphas are known for prodding defenses or exploring areas, or just walking off to the beaten path. Then again, those are all quite human behaviors as well. These creatures seem to have a fondness for melee weapons - but once again, many prudent humans also rely on such.
So yeah, its a reanimate with a survival instinct, near human intelligence, and extremely difficult to tell from a normal person up until it bashed your head in with a hammer. They are thankfully rare, but that certainly doesn't mean you'll only find one at a time.
An Alpha has stats similar to a baseline human - four strength, five wits, four quick, three tech, three ganged, and four close. Some might even qualify for certain skills, though that would be rare. They can run, (speed 10/run) but are not likely to give chase - preferring to reevaluate and ambush the prey at a more opportune time.
Alphas are harder to damage than humans, but not necessarily as oddly tough as their lower ranked Type One - SDI 3, NP2.