This post applies only to those <a large plague rat> found in Befallen.
For an idea of their HPs, I randomly killed 10 of these from all over the zone. One on one, strictly melee damage, went as follows:
1 - 215
2 - 219
3 - 252
4 - 195
5 - 194
6 - 217
7 - 210
8 - 214
9 - 234
10 - 229
So they have on average 220 HPs, give or take depending upon level I assume.