I'm not sure a dog would actually help against Njerps, based on my experience: I had a Njerp who tore down my bear traps around my homestead, so I repaired them while stealthed. I saw something "in the corner of my eye" flicking past, and sure enough, when done and turning around, there were footprints from the bugger passing about 5 tiles away from my character. The dog had been completely silent (this was before the introduction of bones).
The problem is that dogs only react to Njerps when they (the Njerps) aggro, not before. Dogs bark in alarm when robbers close in to demand your stuff, though, as well as at the sight of predators (but their hearing and smell is really poor: a predator can be just a few steps behind a dogs back without it noticing).
Njerps can, and will, destroy traps in trap lines. It's possible to make "mine fields" of traps, but my test of those while fighting Njerps (moving so they'd move into the traps) wasn't encouraging: pit traps with and without stakes caused some minor damage and delayed them slightly, but passing through two or three of them didn't neutralize them. A bear trap rendered a Njerp unconscious for a few turns, after which he broke free and resumed the attacks.
However, in my current game I've found a dead trader in one of my stand alone pit traps (no stakes), and in the game above, with the Njerp burglar, I had an adventurer/woodsman arrive, chop down two trees in my trap array, and then proceed to step into the pit trap that was in between them as he'd broken the line. He claimed to be fine for the two months it took to heal up...
However, it seems Njerps wear less armor now than they did, so they won't tire as quickly, but on the other hand they have less protection against trap damage.
As a final note, I've got a fox trap trap line around my current homestead (I don't really want to trap stuff with it, but just keep birds and other small animals out), but a vagabond woodsman has been observed crossing that line without any issues a lot of times, without disturbing the traps, indicating a Njerp ought to be able to do the same.