It was a monstrous thing, all skittering legs and scything blades. I've seen golems before, proper things made of metal, or stone. They were preferable, because this thing, it was so fast, and it was drawn to out pain, to our screams.
While each Witch Engine is unique, they all share many common features. They are made of commonly available materials, usually a combination of natural wood (branches and tree trunks, not shaped boards or planks), the bones of animals, their corpses, and the corpses of men who have transgressed against the witch. It will have their melee weapons to use as it's own, but in a more crude fashion, such as swords and knives being used as body spikes, or being attacked to arms.
The average Witch Engine is large, eight to twelve feet tall, with a compact core wrapped around a clay enchantment vessel. They will have multiple legs, much like a spider or an insect. Their materials limit how much weigh they can bear, so there are no humanoid witch engines. The most common configuration is a spider like centaur.
Durable - with many legs and a non-biological construction, witch engines are tough and hard to damage. Techniques that work against golems, such as breaking legs, are much less effective against a witch engine.
Fast - being light, compared to other golems, witch engines are fast, often faster than most non-speed based characters. They are generally going to strike first in combat rounds, and are likely going to have multiple attacks within a round. The worst outcome for a character is to be pinned by a witch engine, which allows it to make multiple attacks of opportunity against them. These individual attacks are not devastating, but being stabbed 4 times a round with daggers is going to wound even tough characters.
Frightening - covered in bones, often with theatrically made faces and auras of fear around them, a single Witch Engine is going to be able to easily route low level characters, anything in the under level 5 range (D&D stats here) is going to scattered or slaughtered. Phalanxes of soldiers will break, horses will bolt under their riders, and even stoic men will fall back rather than face the monstrosity.
Hide in Plain Sight - made of natural materials, witch engines are able to hide very easily, lurking underwater, hanging in trees, burrowed into the earth. This makes them phenomenal ambush attackers, guards, and hunters.
Legendary Witch Engines
Witch Engine Ryll - the dragon Ryll was not particularly feared, nor fearsome. It was a medium sized black wyrm that was more inclined to stealing sheep and sleeping in the swamp than harrowing mankind or building gold hoardings. When the wyrm passed few noticed. The corpse was taken and turned into a stunningly dangerous Witch Engine. The engine is 40 feet long, and has the claws, fangs, and wing attacks of a medium sized dragon. It also retains a vestigial breath weapon. Rather than flame, it produces a cone of poison and rot. Anything in the cone rots, or is sickened. Injuries are worsened, and victims are blinded and nauseous. The fear aura of Ryll is much stronger than the average engine, and can scatter up to 8th-10th level characters.
Hellshade - the Witch Engine Hellshade currently is ensconced in the Abyss, as it is made of the corpses of a dozen demons. The witch who made it also dwells in the Abyss, and the engine remains her defender. The abilities of Hellshade are beyond imagination, and include infernal magic, breathing gouts of flame, and even summoning lesser demons to it's bidding. According to the mythos, there is a bound demon lord inside Hellshade, providing it unlimited power and control over other demons.
The Siege Tower - made from the bones of trolls and giants, the Siege Tower is a massive witch engine, that stands tall as the trees, and can break stone and walls with it's massive fists. The witch engine stands as a silent guardian in the middle of a witch wood, and fear of it keeps the treekin in line.