Jefferson Starships 2.0
These are a hybrid of werewolf, wraith, doppelganger, and vampire.
They have a massive weakness to silver, and yet they can become anyone or anything. They have vampire fangs and mostly survive on blood, but can eat other parts under certain circumstances. The werewolf in them can be used to bulk up with a massive strength boost, based on the main strength of whoever they are copying at the moment. Basically, if they were in the form of Andre the Giant and wolf out, it's bad for everyone.
Fortunately, they mostly prefer to avoid detection, and transform to less noticeable forms usually. The wraith allows them to use the darkness to cloak themselves and attack. However, none of these abilities are quite as strong as the pure versions of the creatures, but they are still hard to deal with.
They are virtually undetectable without help from particular substances derived from drugs that make you more perceptible to the world around you. These are usually considered hallucinogens, however.
Do not, under any circumstances, provoke these creatures, or you will most likely die. However, you manage to kill one, do an autopsy. Study up on these.
Also, these are Jefferson Starships 2.0 because they suck so much to deal with. They're 2.0 because the Supernatural version of "Jefferson Starships" are much tamer in comparison.