Equoid

Charlie Stross' most recent Laundry novella Equoid, available on Tor's website, is a characteristically disturbing and subversive take on unicorns. Not for Stross the kindly unicorns of myth; rather his equoids are sexually diamorphic flesh eating monsters with the ability to coerce the local human population into doing their bidding. The story cleverly fuses bits and pieces from marine biology, Cold Comfort Farm, and, finally, explains how a chance encounter with an equiod during his formative years turned young Howard Phillips Lovecraft into damaged misanthrope we know today.

It excellent addition to the Laundry files but only for those with a strong stomach...

ETA: Stross has a post on the history of Equoid on his blog.