Serious discussion time, please.
So, the Monstropedia entry on Shadows wrongly associates Tulpas, Thoughtforms, and Eregores with the theory of negative energies combining to form Shadows and their ilk. I will politely assume someone was rushing on that part.
See, a Thoughtform is a bastardization (Western misinterpretation) of a Tulpa which is a concept in Tibetan Buddhism on bringing an idea to life. A Thoughtform is a Hollywood worthy take on a Tulpa; think of mentally creating/birthing a physical golem. There is a theory that people thinking of entities hard enough or long enough will make them literally real. The Monstropedia entry just tacks this onto its blurb about energies combining to form Shadows.
Also the entry cites Eregores next to Tulpas without any further information. An Eregore is the head honcho of a hive mind or the collective hive mind of a group. An Eregore can be a type of Thoughtform that is also controlling other Thoughtforms or it can be a person or other entity that created a Thoughform (or multiple Thoughtforms) and is able to influence them. That's not a pretty description of it but it's effective I believe in explaining it. More often the term is used to describe collective minds or people who have come together after having the same thought.
Definitions finally aside...
It kinda seems to me that either humans are just hardwired to be unimaginative and that's why we've thought up similar types of apparitions and monsters around the world over the centuries or earlier on people could have had nightmares strong enough to set off the Big Bang of the monster world. Everything has a beginning right? Shadows are the best sort of entity to focus on with this sort of theory since they're so much simpler than the monsters of specific cultures. People everywhere will claim to having seen similar things as a child. Figures made of shadows moving about, slinking in their peripheral at the very least. Other sorts of monsters tend to have some origin associated with them, like improper burials, but Shadows don't have any creation story. We just have guesses like this Thoughtform theory I'm leaning towards.
And finally the discussion prompt: does the Thoughtform theory hold any weight for anyone else? Reasons to disagree? Random thoughts spawned from reading my rant on this theory?
I have a suspicion that a lot of people on here would go with that "negative psychic energy" combining theory... even so I would very much love to hear someone go on at length about why they prefer the energy theory or any other theory over this one. I don't mean to champion this theory it's just the option I'm liking the most so far. This can be your opportunity to try to sway me away if I did a poor job arguing for it to you. : )