Jeez, its been ages since I last played. Evidently the new version seems to have changed a lot. Earlier I used to get around 20 migrants per year. In my current one, I reached 150 dwarves in 4 years (and I was playing quite slowly).
Why the dwarves forgot about the beast
One of em appeared in my fort, a giant humanoid comprised of steam, with huge scary mandibles, and for some reason undulating rhythmically. It scared the shit out of me, because I had absolutely nothing in the way of an army, and the only defence I had in the cavern was a door surrounded by cage traps (to capture the assorted evil building destroyers). It destroyed my nice door, and then ran up to kill my herbalist.....
The Forgotten Beast punches the Herbalist in the upper body, but the attack glances away. (x20 or so)
Eventually the herbalist's kitten joined in the fray and scratched out a couple of his arms. Furthermore, while the dwarves and cat were totally unaffected by the steam, it appeared that the beast itself seemed to get hurt by it from time to time (more WTF). Finally a random peasant kicked him in the head (which promptly flew off).
No wonder it was forgotten.....
My third FB was an angry rhinoceros beetle that emitted deadly gas. I'm not sure about the deadly part. It got raped by the 50 or so slackers I drafted and sent to finish it. However, its true effect was only seen after its death: many of my dwarves now have beast sickness. This essentially means that they are sluggish and leave long trails of vomit wherever they go. Pretty much my entire fort is covered with piles of vomit. Thank Armok you don't get diseases from vomit.
The worst part is that I can't even get my 40+ jobless idiots to clean it up. Gah.
Research on Forgotten Beast
Through a lot of savescumming, I managed to determine that a FB would appear on the 19th of Obsidian in my cavern. I did that multiple times, and noted the beast that appeared.
1. Winged Theropod
2. Rhinoceros Beetle (this is the one I wrote about)
3. Humanoid of solid salt
4. Humanoid of glass with deadly webs
5. Titmouse (its a kind of bird)
6. Humanoid of glass with deadly gas
7. Fire Blob (this one burnt all the grass, destroyed the door and boiled the cage traps. i think it died after inhaling boiling lead.)
After this the theropod appeared again. I can confirm it's the same one because the name was the same too.
Adding the 2 beasts I got earlier, it appears that my "smaller" world seems to have generated 9 FBs.
[UPDATE] Whoops. There's more than that. Theres a giant winged quadruped made up of gypsum plaster invading now.