[Crimsondude]

I just assumed it meant shadow and/or the spirits introduced in Street Magic.

[CanRay]

Shadow Spirits and such, yes.  Horrors?  Not yet.  Not even the minor ones.

Thankfully!

[The Wyrm Ouroboros]

You don't read very much, do you?

Horrors yes.  But they have to be very specifically pulled over and, well, I think I would describe it as 'kept alive'.

[TheNarrator]

Some of the Shadow Spirits seem to have a similar sustenance as Horrors, feeding on metahuman suffering that they caused. And the taint they afflicted on the tempo users so that they could come back for them later is a lot like a Horror Mark. Which makes me wonder if there's truly a difference. And some of the things listed as "Horror Constructs" in Earthdawn are in Shadowrun as "Critters".

Horrors were already entering the world in subtle ways before the start of the Scourge proper. Something killed six monks in the monastery the very night that the first person to read the Books of Harrow killed himself, centuries before the Scourge. Something killed every infant born in the kingdom of Scythia and destroyed the kingdom years before. Bonecrown the Usurper got inside Parlainth before it left our dimension. They'll be here long before we realize they are, just not en masse or in obvious ways. Or possibly even physically; there are purely astral Horrors that may require less mana to exist in our dimension.

[Mirikon]

Horrors need a certain background count in order to come over, yes. But they need a much lower background count to STAY over, even though they won't be as powerful as they would when they could come over naturally. Think of it like a normal spirit in an area with a negative background count. The more powerful the spirit, the higher a negative count it can withstand. However, it would take something equally powerful to be able to draw them over in that state to begin with. Which is why the horrors were using the Great Ghost Dance sites to try and slip through. If a Named horror came over again, like the one chasing Aina did, then it would be able to stick around, at its reduced level. It would likely be mistaken for a shadow spirit, and would stay away from anyone who might recognize it for what it truly is, until it could bring over its friends. Some of the uber-powerful shadow spirits you hear about, such as the ones down by Bogota, could in fact be horrors that have slipped through before Montgomery shut the gate in Hawaii, and are only now getting to a level where they can actively influence events.