This is one of those ideas that prove that you can get ideas anywhere, in this case it is a monster inspired by contemplating beauty treatments.
The Beautifier looks like a giant worm. big enough to swallow a person whole. Its digestive system is not actually strong enough to wholly digest a person, instead its gut enzymes cleanse and rejuvinate the skin.
Since the trip through the digestive tract takes about an hour, a person needs to have some sort of oxygen supply while undergoing the treatment.
There are several ways to use such a creature, it could be a background detail in a SciFi Setting, or a focus of a mystery, as a person is found suffocated with extremely soft and clear skin (somebody whose air supply failed during the treatment, disposed of in order to avoid lawsuits)
In a more post-apocalyptic setting, some of the beautifiers have escaped, and now roam the cities, still trying to beautify people…
Finally, perhaps they are a naturally occurring organism, and the players are hired by a spa in order to catch a couple of them.