When you awake from your sleep with a new thought, did you actually conceive that thought, or did someone or something implant it in your mind?

Story summary: Nachtmahr [German] - Evil spirit from germanic folklore who sits on people’s chest and causes bad dreams.
The Nachtmahr is believed to "ride" on horses, leaving them exhausted and sick in the morning. They are similar to mythical creatures like the Incubus or Succubus.
What's the true meaning of "Nightmare Moon"?

“Nachtmar” by cultpony

Original story: https://www.fimfiction.net/story/226179/nachtmahr

Music: https://www.youtube.com/user/myuuji