Subject: This has been answered before. (Also, spoilers maybe.)
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Posted on: 2014-11-01 23:21:00 UTC

It was in the eleventh Doctor's episode The Doctor's Wife, when the Doctor mentioned an old friend who'd been both a man and a woman. Name of the Corsair.

S/he was apparently mentioned by the fourth Doctor according to the TARDIS Data Core Wiki, though since I'm still catching up on Classic Who, I can't vouch for it personally.

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