If you group down near the bubonians and are doing OK down here, watch out for these things. They took out my decent group in about 25 seconds. Really pissed us all off.
Tough little buggers. Only place I've seen them is in the Bubonian area to the right of zone in. They appear to be "trap" type spawns that spawn several when you randomly step in certain spots.
I've seen anywhere from 2 to 4 spawn on a triggered trap, but those might not be the extremes. Could not single pull even with Bard harmony, but a good Bard or Enchanter mezzing can take care of them.
They seemed to hit about as hard as the rats, but had very few hitpoints.