For the TNG episode, please see Imaginary Friend (episode).
An imaginary friend was a pretend companion that a child would create in their imagination.
As a child, Guinan had an imaginary friend that was a Tarcassian razor beast. According to her, "It had dark brown fur and gold eyes and huge spiny wings, and it would fly so fast nobody could see it except me." (TNG: "Imaginary Friend")