In The Pursuit of the Heiress by A.P.W. Malcolmson, there is a rather curious story about the 3rd Earl of Darnley. Malcolmson writes:
"Another strange tale, which this time ended less happily for the heir presumptive, is that of the 3rd Earl of Darnley, an eccentric bachelor who suffered from the delusion that he was a teapot. In 1766, when he was nearly fifty and had held the family title and estates for almost twenty years, Lord Darnley suddenly and unexpectedly married; and between 1766 and his death in 1781, he fathered at least seven children, in spite of his initial alarm that his spout would come off in the night."
I have no idea how true this is. He's not listed in the Dictionary of National Biography and all the Peerage entries only mention his basic biographical details. Even Wikipedia doesn't get into it. However, according to Futility Closet's entry, Malcomson has three strong sources to support this story.