Dead Heat (Alpha & Omega 4) by Patricia Briggs
Synopsis from Goodreads:
For once, mated werewolves Charles and Anna are not traveling because of Charles’s role as his father’s enforcer. This time, their trip to Arizona is purely personal, as Charles plans to buy Anna a horse for her birthday. Or at least it starts out that way…
Charles and Anna soon discover that a dangerous Fae being is on the loose, replacing human children with simulacrums. The Fae’s cold war with humanity is about to heat up—and Charles and Anna are in the cross fire.