"Diamond Jim" is an unproduced episode of Adventure Time. It was apparently in development in 2009 for season one, and is known from story notes posted here on the Frederator Blog.
From the looks of the story notes, it seems BMO gets its memory erased in it. It also possesses a scene of Lady Rainicorn riding on a horse, which is later used in Jake's 'imagination thought' in "Mortal Folly." The Old Man from the animated short was scheduled to appear in this episode.