Brightlight's "Great Work" in Black Madonna is implied to basically be a modern version of the "kaers" from Earthdawn, or at least it was (if there's any lesson that should be learned from the novels, it's that even immortal elves and other great dragons don't get away with ripping off Lofwyr)
As a general rule, the magic level has to be fairly high for them to appear (IIRC the general level won't be high enough for centuries to come, according to sources in the know). One of the overarching plotlines of the 2050s SR was that there was a threat they could 'leak' through at places where the magic level was high enough, as a sort of 'bridge', deliberate or otherwise. (Considering that the site of the Great Ghost Dance was one, it's ironic that Sam Verner's attempt to fix his disastrous blunder in Find Your Own Truth almost caused another). The Dragonheart trilogy essentially ended that plot arc, but the implication in the Corporate Guide is interesting.
The runners that did participate in Harlequin's Back are mentioned in the Aztlan posts when Dunk and company are discussing where the secrets of 'blood magic' leaked out from (although Harlequin dismisses them as a source on the subject).
The Wild Hunt are definitely not Horrors - they are labelled as 'unique spirits' in Magic in the Shadows.
Harlequin mentions the differences between the insect spirits (invae) and the Horrors in Harlequin's Back - for one, the latter aren't spirits. He labels the insect spirits more as 'nuisiances' whose appearance was hastened by the Dance.