I was on a map with no goblin civilizations to test out some layout ideas I had when a hydra decides to show up.
Although I had my trained dwarven militia, I had completely ignored making any fortifications against intruders. No ballistas, no moat, no drawbridge, nothing at all.
The hydra just waltzed into the fortress and the only thing in its way were the roving band of puppies gathered near the entrance. They tried fighting the hydra but it killed all of the puppies it encountered, but left all the cats for some reason. The hydra then proceeded to go into my great hall, killing every puppy foolish enough to try and stop it.
Incidentally, all of my soldiers had decided to have their meals in one end of the room, and my miners (who married each other) threw a party and invited everyone they knew!
The hydra fed well that day.
Eventually, my little warriors decided to actually fight the hydra, but not until they had a good, long look at watching their fellow dwarves get ripped apart with the only resistance being the puppies. They finally got off their butts after there were so many dead bodies and chunks of various people and puppies, began rotting and filling the whole room with miasma.
By the time my soldiers decided to actually do anything, half my population was dead. Being pragmatists though, the dwarves butchered the puppies they could find that hadn't completely rotted and the hydra corpse and made a lavish roast out of hydra meat and puppy tallow. The last surviving tanner had a lot of work suddenly come his way as well.
The following season, I had a craftsman get a strange mood and build a artifact. It was a gabbo table that used the hydra leather as an accent.
The image was of a hydra killing a puppy.
It seems the dwarves will remember that day forever because almost every image made after that was either the hydra killing dwarves or the hydra killing puppies.