Setup: I embarked on a coastline, just cause I hadn't done so before, so that meant I had to dig down for fresh water. After hitting the caverns, I built a well over the body of water and dropped a scout dog down into the dry area. I then walled everything but the well off until I had the time and resources to develop down there in ernest.
This is where things get interesting.
After a few seasons, an artifact set of native gold earrings and a wave of migrants later, I got rather startled by a message of a Forgotten Beast appearing.
Because I didn't think of getting a screenshot at the time, I can only paraphrase what it looked like - basically, it was a fleshy mass of eyeballs with wings, and a dust attack. Now, this was my first FB EVER, so I was kinda scared. I was further scared when I realized that the thing could fly, and could path to my fort through the well. Oh, and I had only one marksdwarf at the time.
After a few moments of apprehension, it went in a different direction, much to my confusion. Then I realized - it was going after my scout dog. The poor little pooch bolted down a ramp as fast as it could, but the FB was just too fast. The thing caught up to the dog... and promtly exploded.
After the dust settled, the dog was dead (major concussive damage), but so was the FB, having been flung by its own attack against the ceiling. I had to stop and go to work after that, but I'm probably gonna deconstruct the wall so I can go butcher the remains of that FB.