My headcanon is that long, long ago, perhaps even before the unification of the Three Tribes, various parts of what is now Equestria were under the control of unicorn warlords, some of whom were a bit “sparky,” in the Girl Genius sense of the term. They created bizarre life forms for military purposes, or as magical mad science experimentation for its own sake.
 
At least some of these creatures promptly turned on their creators—and probably ate them—then escaped to the Everfree Forest and began breeding. That is why the forest is full of bizarre creatures like Parasprites and Timberwolves that defy everything we know about biology.
 
Image is an image macro combining a screencap from the show and a Mad Scientist Twilight image from http://paradigmpizza.deviantart.com/art/Completely-Mad-335176655 but I am not altogether certain what the conventions are for credit in such an instance.