10 Doctor Who Mysteries Explained Outside The Show
6. What Was The Hybrid?
The Hybrid is arguably the most obtuse series arc Doctor Who has ever had.
It was first mentioned in The Witch’s Familiar, when Davros described a prophecy about a hybrid composed of two warrior races, and suggested these races might be the Daleks and the Time Lords.
The following episodes continued to tease this arc (i.e. the Doctor saying that Ashildr was a hybrid "in a way", after fusing her with a Mire medical kit), but it wasn't until the series finale that we got something more concrete. And even then, fans were left wanting, because all we got was a series of theories suggested by the Doctor and Ashildr.
Could the Hybrid be the Doctor? Could it be Ashildr? Or could it refer to the pairing of the Doctor and Clara - “companions who are willing to push each other to extremes”?
Quizzed in his Doctor Who Magazine column, writer Steven Moffat was more forthcoming, stating that Clara was “the precise motivator that would drive the Doctor to an extreme that was dangerous for all time and space”, and that “the coming of the Hybrid would be the result of their association."
So that settles it. Thank god he didn't canonise the half-human thing...