A reader has drawn my attention to what he thinks is a Third Man appearance in Star Trek. I have not seen the episode and is seems the being in question is not entirely like the Third Man, not a helpful selfless companion. Below is what Wikipedia has to say. If anyone has seen the episode in question let me know what you think...

While visiting Argrathi, Chief O'Brien asks too many questions and is convicted of espionage. Instead of incarcerating convicts, the Argrathi correctional facility gives them memories of years of imprisonment in a few hours of actual time. O'Brien experiences twenty years in jail before his fellow officers can extract him from the situation.

For all but the final week or two of his subjective incarceration, his cellmate is the fictional Ee'char, whom O'Brien eventually kills over a few pieces of bread.

Back on Deep Space Nine, it quickly becomes obvious that O'Brien is not well. He is short-tempered with everyone, almost hits his own child, experiences flashbacks to jail and has full blown psychotic hallucinations. He imagines conversations with Ee'char, who urges him to get help. Despite this, he prepares to commit suicide with a phaser. Doctor Bashir catches him just in time and talks him out of it. A new course of heavy treatment soon has O'Brien on the way to recovery.