Without getting into vamp-baby making 101, let’s just say that the Original witch, Esther, wasn’t entirely faithful to her husband, Mikael. It’s not a story we’ve gotten all the details on, but suffice it to say that when the Originals were living amongst the werewolves 1000 years ago, Esther got pregnant by one of the furry fellows — and that baby turned out to be Klaus.
Klaus’s paternity remained a secret until his mother turned him and his siblings into vampires. The first time Klaus got a little too greedy for human blood and killed his victim, it triggered his werewolfism. Esther placed a curse on her hybrid son to prevent him from harnessing his werewolf side — a curse which was broken in Season 2.
The fact that Klaus is half werewolf has interesting implications now that there’s a cure for vampirism on the table. If Klaus were to take it, he wouldn’t simply be a human again; he’d actually be a werewolf.
Now that there’s an Originals spin-off in the works, we’re hoping to get more of Klaus’s backstory, including an introduction to his wolfish father.