I just (finally) had my first forgotten beast.
A giant hairy grasshopper, with wings of stretched skin that undulated rhythmically.
I was excited, sent my military down to the caves to meet it, but they preferred to hang out in the barracks. Whatever, they'll get the picture soon enough.
So I watch as some war dogs valiantly charge the beast. They all latch onto it and start gnawing at it. The beast kicks one of it's legs and sends one of the dogs flying off a good 20 or so tiles straight into a rockface. The poor dog exploded, it's head, tail, and 2 legs flying off in random directions, and it's blood coating 5 tiles of wall and at least a 3x3 area around the wall, not including the blood that dripped out of it's body parts as they flew off (it's tail landed nearly 10 squares away).
The beast then quickly dispatches the other two dogs (it bit off their heads), and starts chasing a mason that was in the caves. This has to be the most unlucky mason ever. Apparently he was at the moment he ran into the forgotten beast, also running from a cave crocodile. I hoped that the beast and the croc would fight it out, but no luck. The croc took one look at the giant hairy insect and dived into the underground lake, never to be seen again (I assume it swam off the map, as I have no cave croc corpse/meat in my stocks menu).
The grasshopper beast chased my mason around for awhile before my military finally arrived. One of my hammerdwarves got his hammer stuck in the beast's shell and then got tossed across the room. A speardwarf had his arms removed and quickly bled out. Finally one of the dwarves scored a lucky hit, rupturing an artery and causing the beast to bleed out.
I managed to butcher it, and got like 72 units of forgotten beast meat. Plus a HUGE exoskeleton. I wonder if dwarves could use it's exoskeleton as a shell in artifacts, or if it counts as a bone, or what. I'd like to see it used. If nothing else I suppose I can put it on display using a 1-tile stockpile somewhere.
Pretty awesome.