I generally find lifeforms that count on magic creatures like golems or the like common enough that things evolve to prey on them rather unlikely. A better explanation is this a weapon built to destroy such creatures, which opens up the Who and the why questions.
So accepting this is a magic weapon to destroy golems, having a side effect of conversion into monsters is somewhat more plausible, but simply leaving a patch of green where once stood a golem, or a half-consumed golem lashing out blindly are less odd to me :)
Consider the idea of spontaneous genesis of life, as used to be belived. What strange kinds of life might arise from modern substances? What kind of vermin would a landfill produce, or an oil spill on the ocean?