I live not far north of Boston, up against some conservation land. Last night I heard something that, from its movements, sounded about the size of a raccoon but didn't move like a raccoon. It was skittering rapidly through the underbush in a prolonged way. I listened to this cautiously for a couple of seconds. Then it stopped and made a loud, deep grunt like a demented hog. I took one step back and slammed the back door and locked the windows. What was this? I should add that all my life I've spent a lot of time outdoors, day and night, all across the northern US and southern Canada, and I've never heard anything like it except for an actual pig on a farm. Also, I've lived in this house for 8 years and never heard it here before. I'll feel much happier when I can identify it. Thanks for your help!
There are other, less likely possibilities - deer grunt, but they are obviously much larger than what you are describing. Martens and Fishers also make odd noises, and both are known to live in Massachusetts, but they are less common and they tend to scream/shriek rather than grunt.
It's hard to give a definitive answer, but based on your description one possibility could be a badger.
They make some really astonishing noises ranging from grunts to screams to barks. Maybe head over to Youtube and listen to some badger videos and see if that sounds similar?