A ghost or phantom.

“The building was haunted by a couple of spooks.”

A hobgoblin.

A scare or fright.

“The big spider gave me a spook.”

To frighten or make nervous (especially by startling).

“The hunters were spooked when the black cat crossed their path. The movement in the bushes spooked the deer and they ran.”

To become frightened (by something startling).

“The deer spooked at the sound of the dogs.”

To haunt.