They appear as the rotted dead, because they will be dead for way longer than they've ever been alive. They are weak, and do not have the power to act, to kill, to think, really, though they have learned much they don't even realize through passive observation. They are batteries for potential, and show their death in life because of how likely their extended decay is. They will slowly follow and watch but otherwise do not act unless acted upon.
Do not kill the zombies, because you take their existence from one of potential to actual. You break the freshness seal and all that potential comes pouring out.
They are easy enough to kill, and passive, but once their death is certain they spring to 'life.' Their appearance is of a hale member of their breed, dressed for survival and battle. The easy part was killing them. Now you have to fight them, or, rather, you have to fight any them they could have been before they were killed.
You have to survive everything they MIGHT have done to avoid their death.
However many rounds it takes to kill them or whatever the level of the being who killed them (whichever is higher), they leach this to retain a toehold in the world for that number of Rounds. Their abilities are mirrored in your abilities. If you stab them they emerge mighty warriors. If you cast a spell on them or Turned them and so blew up their potential shambling eternity they emerge mighty spellcasters. If you backstabbed them then each new assault of theirs is more surprising than the last. Etc.
They only have access to the spells or combat bonuses you would at your level, but here's the sad trombone: they are all inevitable. No roll to-hit. No save. Your save and AC against all the creature's future attacks is the creature's saves and AC when you attacked it. Because you have rendered their death inevitable, you have locked in a track of possible, and until they use it all up anything POSSIBLE for them WILL happen. They can still run out of spells or arrows and shit but otherwise they are a story already written by you.
When they run out of Actuality to sustain them and drive their vengeance, they slump as dead, or disintegrate, or look like a healthy alive person you just killed somehow basically.
This can be a shitty gotcha trap but it is a fair shitty gotcha trap. While you may not recognize the nature of this threat before engaging, should you survive your encounter you will ALWAYS know these creatures on sight, which lets you avoid them or, oh goody, weaponize them. You can also try to just lead them into open pits but 50/50 they appear behind you and push you in, too.
Whence they came some say A Wizard Did. It's possible this is a kind of spell effect you can learn, a curse you can put on your enemies in order to turn them into Suicide Successes for your cause, or just to guard your cool walking house when you're not home. Maybe this is just a thing that happens in the universe, in which case nobody knows where new abominations (all seeming dead and rotted immediately, about 30% becoming adults on the death precipice immediately) even come from.
...unless death lets them fuck.