Cephalopoid

The Cephalopoids are a sapient extraterrestrial species, some of which are known to live hidden on planet Earth. They are capable of high speed and agility as well as the ability to climb vertical surfaces bare-handed. While their true appearance is not known, they have ocular gill sets which appear like vertical eyelids, blinking occasionally, presumably when they go short of oxygen.

Behind the scenes
The Cephalopoid's true appearance has not been shown in the movie, but based on their name they are likely to be mollusk-like. It is possible that this is the same of the unnamed species to which Reg and his family belong. However Reg's child is seen blinking with a set of inner eyelids which look different from those of the Cephalopoid, making this idea unlikely.