Equoid (Laundry Files, #2.9) by Charles Stross

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…

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