A small Belgian dandy with a waxed moustache, no beard, and pince-nez

Hercule Poirot of the series of books is a very short man.

Captain Arthur Hastings's first description of Poirot:

He was hardly more than five feet four inches but carried himself with great dignity. His head was exactly the shape of an egg, and he always perched it a little on one side. His moustache was very stiff and military. Even if everything on his face was covered, the tips of moustache and the pink-tipped nose would be visible. 

The neatness of his attire was almost incredible; I believe a speck of dust would have caused him more pain than a bullet wound. Yet this quaint dandified little man who, I was sorry to see, now limped badly, had been in his time one of the most celebrated members of the Belgian police.

Agatha Christie's initial description of Poirot in The Murder on the Orient Express:

By the step leading up into the sleeping-car stood a young French lieutenant, resplendent in uniform, conversing with a small man [Hercule Poirot] muffled up to the ears of whom nothing was visible but a pink-tipped nose and the two points of an upward-curled moustache.

He has all the same been played by several actors notably taller than that.

At 5'7", even David Suchet was a bit too large, though clearly the best (he and Ustinov) at catching the humorous fussiness and dandyism of the character.

Poorly cast in every conceivable way were Kenneth Branagh, Albert Finney, and Orson Welles.

The ABC Murders is one of Agatha Christie's most famous works, published in 1936 and set in 1933.

The book, like its detective, has a comic side, and most film or TV versions of it have been considerably less dark than the excellent, if not very faithful, 2018 BBC version currently shown on Amazon video starring the six footer, John Malkovich.

Particularly apropos to the age of Brexit, reemerging nationalisms, and Trump are the signs of menacing xenophobia BBC has put into this one.

Why does Amazon describe this as a "prime original"?

Why does this BBC version have the actors refer to the unknown killer as they or them in a story taking place in England in 1933, though they think of him as a single unknown person?

Neither Agatha Christie nor anyone else in 1936 - or 1933 - used singular they.

"I will find this person, Madame. I will find them," Poirot in this video says to the dead body of the first viction, Mrs. Asher in Andover.

And so on from there, again and again.