These spirits are usually in places where a Merchants or sketti mistahs made a successful exchange, they take what was exchanged from the fluffy when they are asleep, is one of the only spirits known among fluffies strong enough to interact and cause biological changes.
To take a body part that was exchanged they seal shut the blood vessels leading to said part, causing necrosis on the specified part or parts, there have been fluffies pillowed because of this necrosis, leading to almost immediate regret and pleading to have their body parts back.
For foals the Decay itself will make the entire foal enter in a estate of necrosis at different times, leaving only a putrid foal for the parents who exchanged them for sketties.
The presence of a “feral” Decay also causes makeshift fluffy nest’s structures to weaken and collapse faster, usually having the families inside or collapsing during the cold times.