Funny, I saw the thread title and my first thought was "alligators."
I have lost several embarks immediately to alligators before, and in a few recent embarks I've lost some dogs and watched my fisherdwarf get rekt by an alligator in the turtle pond he was fishing in.
I've also lost a couple of embarks losing my only pick when my miner dies while plugging an aquifer. Usually on aquifer spawns I only have a single miner and focus more of my early game into above-ground stuff, and I've since started bringing an extra pick. Maybe I should learn how to dual-channel.
As for recent quick deaths, I had a strange first experience with blind cave ogres recently. I was constructing cavern entrances, as I was designing a fortress to be more focused on obtaining plants, wood and meat from the caverns than from the surface world, and in the process of setting up some cage traps around the entrances I noticed three blind cave ogres just, standing there. My dwarves didn't seem to mind them and they didn't seem too hostile, so I just locked the door and decided to deal with it later.
When going back down to the z level they were on, I was alarmed to notice that the door had been broken down, and two were standing inside my fort, while a third had been caught in a cage trap. Once again, dwarves were walking all over right next to them, and I assumed that they simply weren't a problem and I could go about my business and deal with them later.
Then I guess they killed a couple of dogs up by my meeting hall, and instantly half of my dwarves entered combat with one of them and got stuck in over-exertion mode. I attempted to bury the ogre by channeling the floor underneath it, and my dwarves followed it into the 1x1 pit I had designed for it. All the while the other ogre is just standing around doing nothing in the same room. Was quite funny really.