Spoiler Alert—My Players, skip this post to maintain maximum fun
The giant weasel is a mean 3+3 HD monster and with its special drain blood attack it should have a low or medium level party thinking twice about crossing them. That is, were it not for its pelt. The pelt, worth between 1000 and 6000 GP, is probably the most valuable treasure any 3HD monster will ever carry, and once your players find this out (and find a furrier who can pay what they're worth) they will be willing to lay a lot on the line for a chance at such booty, and as they advance they may hunt them to extinction as best they can.
The Monster Manual has weasels commonly (~20% likely) guard kobold layers, and, again, if the players catch wind, this fact may have them storming kobold layers long past second or third level. On the one hand this will increase the utility of your kobold miniatures a good measure. However, if you're the type who worries about "balance" and going "Monty Haul" then you need to make some considerations.
In my own game the Challengers faced off against several weasels and a pack of kobolds and pulled off the win without losing a man. And they were savvy enough to take the pelts to the nearby furrier they had heard a rumor about. Sure enough, they then put off long term goals in favor of a kobold hunt close to to home. They haven't managed to come to grips with any more weasels since that first encounter but any kobold gang around the corner means another chance of weasel guards. The question remains, though, whether, if and when they do clash, it will be the windfall the players are hoping for or more trouble (death) than the weasels' potential worth.
And on the question of balance, my approach is that if the players get too single-minded I'd be remiss not to leverage the concept of an upset equilibrium and introduce believable (and yet, hopefully, fun to play) challenges that would arise as a consequence. For example, other, likely more seasoned adventurers would catch wind of the opportunity and edge in on the PC's territory. Or the kobolds or another enterprising monster might exploit the PCs desire for weasels and use them as bait. Or they succeed despite these obstacles, hunt weasels to extinction. Again, problem solved. Overall I'm not too concerned about it.
