I was thinking of something akin to the Baku of japanese mythology, a creature able to eat people's dreams. If something like this existed, how would it be able to do that (or at least something similar)?
Since all brain activity is electro-chemical in nature, what you are describing would be a parasite which has breached the blood/brain barrier and is feeding either on the chemicals neurons use to fire, or directly off the electrical energy transmitted though the nervous structure of the brain.
Since much of the electrochemical activity of the brain is driven by charge carrying ions, this might not be the actual "food" of the parasite (a much more realistic parasite would simply absorb the oxygen and sugars in the blood), but perhaps the ingestion of these charge carrying ions is the means for the parasite to regulate its own metabolism, reproduce and so on. High brain activity associated with thinking, dreaming and so on would of course be the prime time for the parasite to absorb these ions for its own metabolic processes.
Given the scale of the neurons in the brain, the parasite would essentially be similar in size and scope to a bacterial colony lodged in the spaces between the neurons. While monstrous in concept and activity, it would not really be a "monster" in the traditional sense of the term.
The part that needs the most handwaving is that the creature is telepathic, and it feeds upon psionic energy.
This energy is transmissible in the same ways as sound, radio, or electrical signals in a wire. The creature can only feed on energy in some specific channels, or modulated by amplitude or frequency, or whatever other means of filtering signal from noise you can think of.
Here comes the dream part. The creature is making use of the Ganzfeld effect, more specifically the way it is used in the Ganzfeld experiment. Therefore it needs its user to be in a dream state in order to feed.
The creature just might be able to collect food from the mind of someone in a trance, or in a state of deep concentration or hypnosis. Inducing sleep or such other states into a person (or animal) would be the creature's way to 'hunt'.