A Namegiver is basically any member of a sentient species. So humans, orks, merrows, sasquatches, dragons, pixies, and so on would all be Namegivers.
Earthdawn magic wasn't really dangerous in and of itself. It was just more powerful and varied because the magic level was higher and they had a longer background of using and researching it. Blood magic, especially in the form of charms, was more accepted and fairly widely used. The Horrors came, not because of anything Namegivers were doing in particular, but simply because the mana level got high enough for them to push through into our world.
And gnashers would probably be the first Horrors to appear. They're weak (relatively), simple minded eating machines that kill and eat everything they come across. They'll even eat rocks and trees if nothing else is around. Basically, the weaker Horrors show up first--kreescra, mindslugs, the aforementioned gnashers, etc--followed by ever more powerful Horrors as the magic level rises. They would probably show up most often around areas aspected to negative emotions most often. So toxic zones, serial killer haunts, and the sites of concentration camps and mass murders would attract them.