The game probably puts the most vicious creature possible in your current biome or something like that.
Then again, it can get really hard to tell what wildlife is on the map at any given moment.
I'd frankly call bull on that. I got most recently "ere the harpies get hungry" or whatnot. Nasty though they are, I have YET to have a harpy take down a champion wrestler and perma-incapacitate another one whilst fending off THREE champion wrestlers. Ogres, though... Those'll mess you up. The main problem with harpies for me is that the bastards keep on EATING my dwarfs whilst they're constructing guard towers... And then they're flying down the constructed stairwells to the center of my damned fort to eat the REST of my dwarfs. It's playing hell to station dwarves everywhere, and besides keeping up food and booze for the lot of these people -WITHOUT a farmer getting carried off by one of these Oz-monkeys- and generating enough rock toys to fulfill the wishes of the crazy freakin' king who'll be giving me meat and seeds and armor and weaponry and bolts, I've got to worry about these sonsaducks. It's playing havoc to have twelve wrestling champions patrolling the perimeter, the towers getting constructed, the interior, wherever I'm currently logging, and THEN still having a champion wrestler-trained marksdwarves response team for those orges who WILL annihilate my hunters, loggers, and fishers.
And I haven't even hit the GCS that's in the unrevealed chasm/bottomless pit, the underground river, or the HFS. And it's all in a terrifying/savage biome. With a volcano. Good news is, I'm a DF player, so I find this entertaining as all hell!
So, uh... I think it randomly selects a potential monster on your map that's nasty in any one of the biomes where you've embarked to.