It appears to be a living, menacing shadow. It is briefly shown against one of the walls of its cube prison, changing forms.
Shadows have different meanings in horror. Sometimes they represent an evil twin towards a character, or they themselves are a creature or a certain form a creature takes. Examples of shadows as living beings include the 1990 film Ghost, Bram Stoker's Dracula, The Princess and the Frog and Nosferatu. Shadows in their own right are a staple in horror even if they aren't living beings. Shadows add a lot to horror in keeping the dark atmosphere and eerie setting.