The following is a list of types of spirits (from strongest to weakest), what kinds of ponies can see them, and how rare they are on a scale of one to five, (five being the rarest).
This only applies to my story, King of Monsters.

Rank 1, Avatars.
These are physical manifestation of a concept. (Ex: Combat, Peace, Worry, a Forest.)
As these spirits are physical manifestations, they can be seen by all.
Rarity: 5.

Rank 2, Elementals.