Shadow people (not the movie)
Shadow people move around by pretending to be someone's shadow. They fix themselves to the soles of your feet. If you already have a shadow they will overlay it. They carefully copy your every movement.
In fact they are not people at all but they are creatures. They are only a few millimetres thick and, of course, dark in colour although they can change colour somewhat to allow for local conditions.
Usually they manage to move around undetected in a human world but when detected humans detest them and try to stamp them out.
The population is small when compared with humans - they can transfer from one person to another in order to get to where they want. Over thousands of years they have evolved to become better and better at emulating real shadows. This is not surprising given the immense selective pressure against them.
Shadow people tend to live in areas where normally there are clearly defined and predictable shadows.
Given that they have to eat several times a day, how can we explain their survival during long periods of cloudy weather when real shadows are ill-defined or non-existent?
Despite their origins as scavengers around human encampments, they have evolved to eat by enveloping waste food scraps particularly around fast-food outlets. They are now town and city dwellers.
They can't change their mass but they can extend themselves (rather like an amoeba does) and become thicker within reason - say when shadowing a small person or animal. They could in an emergency fold themselves but not hold that position for long. They can support themselves on upright surfaces if that is where the shadow would be.
They are considered unlucky and, in any case, it is pretty creepy to have a creature literally shadowing you.
While they eat scraps, they defecate black slime - usually under furniture - so this makes them very unpopular, even for those who aren't superstitious.
The most effective way to kill them is to throw petrol on them and set them alight. Of course you have to be careful where you do this. It's not recommended in a city! They are pretty immune to simple stamping with feet as you might expect. If hit with a sizeable hammer you will destroy where you hit but they can flow back to fill in the hole. They can be cut in pieces but each piece tends to grow into a new one. They are difficult to trap because they can slide up walls however at least part of them has to be touching the ground.
Please ask for other clarifications.