Dark Guardians by Theresa Jacobs
By the time Stuart was a teenager he'd stopped telling people about the imaginary dogs surrounding them.
No one ever believed him. He knew he wasn't crazy. Only sometimes he thought maybe he was. Until one day he meets a woman who sees them too. Finally happy to have confirmation of the creature's validity, he thinks maybe now life can change.
Except she wants to enlist him to kill people, perhaps she is not the friend to have after all and he must make a choice.