I embarked on a map with a brook, a volcano, and a dead volcano once. I got there and noticed the map was crawling with trolls, all hanging around the extinct volcano.
So I set about setting a channel to...redirect the brook towards more productive purposes (namely, flooding the part of the map with the trolls).
It doesn't kill them, but they all decide to wander away from the path of the water, and stay far, far away from me.
Until I notice, a season later, a large red T icon (t for troll) outside my base front door. I check around - none of my dwarves are unhappy due to being ttacked. What the heck killed this? I then notice that the lone kitten I embarked with is injured.
Thats not the worst thing I've seen however. If you stop worldgen at year 5 or 6, you'll see things like, "<Somename> the Bronze Collosses was struck down by <Somename> the ammonite."
(For those of you without the prehistoria mod, pixies and perch and turtles work, too)
For those of you who don't realize how sad this is, this is an ammonite:
[CREATURE:AMMONITE]
[NAME:ammonite:ammonites:ammonite]
[TILE:'a'][COLOR:7:0:1]
[VERMIN_FISH][NATURAL]
[POPULATION_NUMBER:25:50]
[MODVALUE:2]
[AQUATIC][SMALL_REMAINS][FISHITEM][NOBONES][NOSKULL][IMMOBILE_LAND][UNDERSWIM][COOKABLE_LIVE][HASSHELL]
[NO_SLEEP][NO_DRINK]
[NOT_BUTCHERABLE]
[PREFSTRING:intricate shells]
[ALL_ACTIVE]
[BIOME_SUBTERRANEAN_WATER]
[BIOME_ANY_LAKE]
[BIOME_ANY_OCEAN]
[STANDARD_FLESH]
Megabeasts are so pathetic in worldgen.