In the Star Wars Legends novel "Legacy of the Force Book 3: Tempest", the bounty hunter Nashtah (later revealed to be Aurra Sing) describes her parents thusly:
"My mother was human. My father was a ghost in the night, and I doubt even my mother knew his species - but it was obviously a long-lived one." Nashtah drew her lips back in an indifferent smile. "If I ever find out who he was, perhaps I’ll be able to hunt him down and kill him."
She then proceeds to drain the blood from her raw steaks and drink it, stating earlier that her species has an unusual... metabolism.
Her mother is known to be Aunuanna, a human woman seen on Nar Shaddaa prior to The Clone Wars, but I haven't seen her father's identity revealed anywhere. I read somewhere that he was possibly an Anzati, but Anzati drink life essence, not blood. They ARE extremely long-lived, however, often living for several centuries if not millenia, and they also tend to have pale grayish skin - both points in favor of the Anzati theory.
Has the identity or species of Aurra Sing's father been revealed in any canon? I'll take Legends canon, Disney canon, the RPG - pretty much anything goes.