Prisoner Zero
Prisoner Zero is the name given to the alien that resembles a giant hagfish in its natural form, but is a shape shifter who has escaped from Atraxi prison. Prisoner Zero changed form by induced its victims into a coma and then reading their thoughts. It can also change shape into multiple beings that are connected together, such as a man and his dog (see picture), though the voice of the host may come out of the wrong mouth (as seen in the man and dog appearance, when the mans voice came out of the dogs mouth).
Prisoner Zero is on the run from his home planet and it endangers Earth when he escapes to it as its pursuers threaten to burn Earth if Prisoner Zero does not surrender to them. To escape, Prisoner Zero assumes the form of the Eleventh Doctor's companion, Amy Pond, and begins reading her thoughts, but she has seen Prisoner Zero in its true form (see picture).
Prisoner Zero is forced to assume its true form when Amy thinks of it and is caught by the Atraxi after its identification, though it does warn the Doctor of "the Silence" before being taken by the Atraxi.