Redemption — Frances Stockton

They have lived for centuries. An ancient, secretive race of were-panthers faced with near extinction. To assure their survival they must conceal their identities and abilities, while protecting mankind from an unimaginable, primordial evil.
Redemption: The Abcynians, Book III
Her ship overtaken by pirates, Jocelyn Kincaid is rescued by Captain Don Adriano de Montoya. Initially afraid of the privateer sailors call El Fantasma, she discovers he is not the ghost some have portrayed him to be. He claims to be part of a mythical race -- and her mate! Fascinated, she's brokenhearted when he makes plans to sail her home to Boston.
Haunted by the memory of his murdered family, Adriano has sworn to ruin the man he holds responsible for their deaths. When he unexpectedly finds his mate aboard an enemy ship, he saves her life. Despite their unprecedented attraction, he keeps his distance until he learns a secret about Jocelyn that could potentially endanger her and Abcynian kind.
Determined to keep the woman he loves safe, his mission changes. But when she discovers the truth he's guarding, will she forgive him, or set him on a path of destruction?