Twilight's book of Equestria's occult aberrations

Twilight reads a monster hunter's book on the occult entities he had encountered through his time hunting regular beasts

Twilight decides to take the journal she and Spike received by accident in the library and goes into deeper recounting of the 'occult' entities detailed within it.

Disclaimer: Horror tag is for the potential spookee skelytans in the journal.
Additionally, this isn't a sequel, but a focus on another type of monster.