Anyone here read the Heralds of Valdemar? During the Mage Storms, they had spots where the waves of magic energy in some other plane crossed and caused "change circles" in the normal world. Two change circles (spheres, in truth) happening at the same time would switch places (teleport), others would suddenly change the season in that spot, the worst of them would cause and sudden and chaotic magic mutation to anything there. Usually making an unspeakable and indescribable monster. The monsters that were noticed were usually very dangerous, some of them being the equivalent of maga-beasts or semi-mega-beasts from this game (also assorted plants that shoot poison darts and such). The changes would be random though, sometimes you would get something harmless, like a squirrel being twisted into a general shape of a blimp-pig and turning it's fur bright purple, some very large and disproportionate wheat plants that are inedible but a good bug repellent, a animal being rooted to the ground and screaming eternally, or a tree being able to read thoughts and move to avoid woodcutters.
I think these mutated areas would be created by (multiple) large disperses of chaotic magical energy, similarly to the way in the books. But I don't think many positive or beneficial things should come out of these areas (the change spheres were usually equally good and bad if you just take the right precautions), as that just doesn't fit with the style of Dwarf Fortress. These would be an added injury after some sort of incredible magical disaster. Perhaps have the good to bad ratio at 1/8 or 1/12. Pretty much, the event would cause spheres on the map to change or switch somehow, and there would be just enough of them to get some animals or dwarves. And you would get monsters with scrambled body parts, such as a dwarf and a horse in the same change sphere would make a...utterly twisted monster that only resembles either slightly. The change sphere would favor "combine" and "adding body parts" over "tear appart" and "maim", for added fun. You would not want a pack of wolves to get caught in one of these spheres...