In my current fort, I dug into the new subterranean features asap. But something caused this to be interrupted. I noticed preserved meat that I did not produce on the stocks screen, and zoomed down to it. The cursor was on a deep, hidden tile. So I decided that would be the route by which I went down. This uncovered a small camp of Serpent men. They couldn't get to me, because the downstair was above a tile occupied by meat and floor. So for ages I try getting a marksman ready to shoot them.
Before that can happen, a forgotten beast arrives; Arel Enirshislug: A great eyeless crab. It had a short horn and it undulated rythmically. It's midnight blue exoskeleton was waxy. Once one was wary of its deadly spittle!
It was on the level the serpent men were on, and so instead of attacking me (who it could not reach) it attacked the serpent men. I did not see the whole of the fight. The snakes won in the end, with losses. Somehow, though, when I checked the unit screen, one of the serpentwoman spearmen (I didn't write the professions) was listed as friendly instead of hostile. And periodically, I would find one of the more powerful of their warriors to have died. Eventually I eliminate the rest with marksdwarves as I dig into the area properly. And now, the serpent woman spearman periodically comes up to my well for the "clean self" job, then returns to her former camp.
What happened is a mystery. Was the friendly a newborn that grew quickly and took up the spear (goblins born on site can be friendly, even when born of invaders)? A different bug, of unknown qualities? None can say.
But I can say I appreciate the extra guard.