Equoid (Laundry Files #2.75)

Charles Stross's "Equoid" is a dark fantasy tale from the Hugo-winning author of Rule 34, Halting State, The Atrocity Archives, and many others.

The "Laundry" is Britain's super-secret agency devoted to protecting the realm from the supernatural horrors that menace it. Now Bob Howard, Laundry agent, must travel to the quiet English countryside to deal with an outbreak of one of the worst horrors imaginable. For, as it turns out, unicorns are real. They're also ravenous killers from beyond spacetime...