Back in 2008, on probably my third or fourth fortress.
I had a map with my fortress on the right side and a hippo-infested river running north-south on the left side. I built a wall around the fortress and had the bright idea to run an underground water canal from the river to underneath the fortress as a source of fresh water.
I was very thorough, hooked up levers and installed a grate on the river end to keep out the hippos. Worked everything out so there'd be no flooding. Unfortunately, a couple dwarves got trapped so I had to build a temporary access staircase halfway down the length of the tunnel so they could escape. Before I could reseal it, a hippo somehow wandered in and took the tunnel straight into the middle of my fortress.
It was a bloodbath. It just sat in the middle of my main hallway, casually pulling every limb off of any dwarf within reach. My poorly equipped military threw themselves at it, one dwarf after another, but could barely scratch the thing before being converted into a pile of bloody limbs and torsos. It eventually got so fatigued that it started passing out, waking up occasionally to kill more dwarves. After it had singlehandedly halved my dwarf population, it finally bled to death.
Then another one showed up.
Let's just say hippos and dying dwarves were a major theme on the dining room walls. The two hippos gained the names Nastymeal and Perfectfeast. They're immortal figures of legend in my Dwarf Fortress history.