Having fun in my terrifying jungle/untamed swamp. I'm limiting myself to 50 dwarves, and had planned out a mini-fortress below ground in the shape of an 11x11x11 cube. I hit the first cavern seven levels down, so I decided it was time to learn how to build towers as well. After 1.5 years I had 40 dwarves, with 8 of them in bronze armor, and two floors of bedrooms above ground. Building was slowed by the periodic appearance of herds of skeletal elephants, frightening the masons as they tried to add more levels to the tower. I decided I wasn't going to challenge them until my militia had trained for another year.
Then I get my first-ever forgotten beast - a giant 3-eyed monitor lizard with a taste for blood. No problem, it's safely walled off in the first cavern, with my fortress sitting off to one side above a massive lake. I mentally add it to the "list of things to kill in a year", and go back to building the next bedroom level. Suddenly I see the little red "C" combat indicator. The forgotten beast is sitting on the lowest level of my fortress, kicking a dog! This is when I learn that up/down stairs penetrate to the level below, and if that level is open air above an underground lake, then anything flying can get in.
The forgotten beast proceeds to scream up my central staircase, going past stockpiles, workshops, dining hall, barracks, hospital, the lot. It finally stops on my construction level, interrupting a lye-maker as she churns out rock blocks. My militia scrambles to catch up, and after some confused hacking and slashing my best axedwarf settles things by taking off its head. The lye-maker escapes with a torn cheek (she was shaken around BY HER CHEEK), and a novice cook hastily prepares 198 forgotten beast lung biscuits and 46 forgotten beast intestine biscuits - tasty! His boss is still trying to render 104 units of fat in the other kitchen, and the 136 bones get used in a gold throne that menaces with spikes of forgotten beast bone and snailman shell. That's a LOT of spikes.
Roll on to two years, and my first goblin ambush party appears. Good, my militia are ready to be tested! Only... why are the goblins out there on the edge of the map... where the... SKELETAL ELEPHANTS ARE. Oh, this could be GOOD. Six goblins take on seven skeletal elephants in a battle to see who's baddest. The goblins soon learn that while maces and hammers are great at shattering bones, that doesn't do you any good when the owners of said bones don't feel pain and can still stomp you to death with four broken limbs. Reading the combat reports, the lesson when fighting elephants is NEVER FALL OVER. Once you're down, you're basically dead meat. One goblin gets its head caved in by a flying elephant drop-kick, five are put on the ground and then kicked to death in various brutal and unpleasant ways, but one seems set to survive. She's only bruised, dodging like crazy, shattering skeletal limbs left and right. Then she dodges once too often and finds herself at the bottom of a muddy pool. Two elephants jump in to kick her some more while she drowns.
Then a second goblin ambush party pops up, with six axemen led a crossbowman. This should be a fairer fight. Things are touch and go for a while, as one axeman is quickly kicked unconscious. Then his squad-mates decapitate two elephants, and the tables are turned. There's a flurry of elephant-parts, and three more go down. But just as the last two elephants are being brutalized, the goblins suddenly turn and run! Their crossbowman-leader has been kicked unconscious, and they've lost hope. He comes around and starts limping towards the map edge, but the last two skeletal elephants are remorseless. Pausing briefly to finish off the unconscious axeman, they lurch after the crossbowman. They're both missing a foot and their trunk, they have red wounds all over, and one has lost both its tusks - it's basically two cripples chasing an invalid. Whenever the crossbowman gives in to pain and falls over, the elephants catch up and kick him some more, so he gets up and staggers a little further. At last a kick to the chest bruises his heart, and its all over, five tiles from the map edge. Final score: skeletal elephants 8, goblins 5.
tl;dr version: organic forgotten beasts are hell on kitchen staff, skeletal elephants > goblins, and DON'T GO OUTSIDE