Last time on creating creatures: It doesn't matter because this is a new entry! Seriously though, read the other one.
This time: Specters, spirits, and demons.

Ghosts and other ethereal monsters have been at the forefront of mythology for much of human history, and most likely Equestrian history as well. We envision the worst in ourselves in a spectral form because that is how emotions are perceived-- an incorporeal and irrational in the physical world, but still a compelling force to us.

When we write such spiritual creatures for Equestrians we need to keep in mind two vital things: the presence of magic and the ponies' belief system in regards to the dead. Magic is important because in a story it offers insight into what is happening in the real world, exposing more about the monster itself. But belief systems for ponies are vital to constructing new myths and stories, for it is from the pony point of view that the stories are told.

Depending on your theme, the nature of the specter can either be a magical cause, mystical cause (beliefs and myths), or be left in obscurity for the reader to interpret. Either way, there must be a link between the specter, spirit, or demon and the traits of ponies. For example, take the mournwraith from one of my own stories; the mournwraith--like all specters in its lore--is a monster made of magical energy that is drawn to those who died grieving. However, the belief of the ponies who live near one is that the mournwraith is the restless spirit of the dead. Don't use that, of course, since I know you can come up with your own specters.

Additionally, there can be demons or specters that represent ideas in a pony society. The wendigos in Equestria, not the cannibalistic kind we have, embody strife and winter. Like those things from Game of Thrones. So, when creating a demon or specter, the myth surrounding it can be linked to whatever good or bad concept such as birth or war or sickness or ignorance. Whatever it is, just don't make it death. I mean, historically there are a lot of death-related spirits, but seriously, a ghost representing death? Come on.