Dreamcrabs (aka. headcrabs) are the main villain and arch-enemy of Clara Who.
They made their feature-film debut in the 1998 episode Half-Life starring a mute, future, half-human (on his mother's side) incarnation of the Doctor. They would again appear in the 2004 sequel to the original serial, Half-Life 2, and again in the 2010 episode Inception starring the Leonardo DiCapaldi Doctor. Most recently featured in the episode Tangerine.