Albus Dumbledore and J.K. Rowling have two different opinions on that. Let us start with the books.
SECTION 1: Albus Dumbledore
Albus Dumbledore clearly tells us, that Nagini was created by killing an old Muggle male:
" (...) I am sure that he was intending to make his final Horcrux with your death. As we know, he failed. After an interval of some years, however, he used Nagini to kill an old Muggle man, and it might then have occurred to him to turn her into his last Horcrux. She underlines the Slytherin connection, which enhances Lord Voldemorts mystique; I think he is perhaps as fond of her as he can be of anything; he certainly likes to keep her close, and he seems to have an unusual amount of control over her, even for a Parselmouth."
Harry Potter and the Half-Blood Prince; 23rd Chapter; Albus Dumbledore
I'm just wondering: who is the old Muggle man who had to be killed? As far as I understand it, he tried to make Harry's death his 7th Horcrux. But then he was defeated because of Lilly Potters sacrifice. He needs a wand to use powerful magical spells:
"I was ripped from my body, I was less than spirit, less than the meanest ghost (...) Nevertheless, I was as powerless as the weakest creature alive, and without the means to help myself . . . for I had no body, and every spell that might have helped me required the use of a wand. . . ."
Harry Potter and the Goblet of Fire; Chapter 33 (The Death Eaters); Voldemort talking to his Death Eaters.
So, as far as I see it, he had to make the Horcrux while he was Quirrel or after he used black magic to create an interim body.
So I took on the Priori Incantatem section:
"(...)then something much larger began to blossom from Voldemort’s wand tip, a great, grayish something, that looked as though it were made of the solidest, densest smoke. . . . It was a head . . . now a chest and arms . . . the torso of Cedric Diggory (...) More screams of pain from the wand . . . and then something else emerged from its tip . . . the dense shadow of a second head, quickly followed by arms and torso . . . an old man (...) “He was a real wizard, then?” the old man said (...) The shadow of Bertha Jorkins (arrived) (...) The smoky shadow of a young woman with long hair fell to the ground as Bertha had done, straightened up, and looked at him . . . and Harry, his arms shaking madly now, looked back into the ghostly face of his mother. Harry Potter and the Goblet of Fire; Chapter 34 (Priori Incantatem); Voldemort and Harry Potter in a battle
So the order of death by Voldemorts wand started with the last:
- Cedric Diggory
- Old Muggle Man
- Bertha Jorkins
- Harry's Parents
So I guess that he old Muggle man (first quote) is Frank Bryce:
"Frank Bryce was the Riddles’ gardener. He lived alone in a rundown cottage on the grounds of the Riddle House. Frank had come back from the war with a very stiff leg and a great dislike of crowds and loud noises, and had been working for the Riddles ever since (...) Wormtail beckoned Frank into the room (...) And then the chair was facing Frank, and he saw what was sitting in it. (...) he never heard the words the thing in the chair spoke as it raised a wand. There was a flash of green light, a rushing sound, and Frank Bryce crumpled. He was dead before he hit the floor."
Harry Potter and the Goblet of Fire; Chapter 1 (The Riddle House); Voldemort killing Frank Bryce
So is it possible - or even certain - that the Frank Bryce is the man who was killed to make Nagini a Horcrux?
SECTION 2: J.K. Rowling
J. K. Rowling, on the other hand, tells us that Nagini became a Horcrux after killing Bertha Jorkins:
J.K. Rowling: The diary – Moaning Myrtle. The cup – Hepzibah Smith, the previous owner. The locket – a Muggle tramp. Nagini – Bertha Jorkins (Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued).
Bertha Jorkins is not old, not a Muggle and not a male. While man also can refer to a human on English, she would still not be an old Muggle.
SECTION 1: Conclusion and Final Question
Albus Dumbledore and J.K. Rowling tell us two different things. While Dumbledore might have been wrong on this, he sounds very sure; in the 6th book, he point out where he is sure and factual and where he guess. I am very sure that Albus Dumbledore is right on this.
Bertha Jorkins, on the other hand, won't make sense at all! While he killed her, he still hadn't a body yet (it was first summoned some time after he killed Bertha Jorkins). And it wasn't Nagini killing her. Not to mention that it would take very long time to prepare to create a body for Tom Riddle; and as far as I know, you have to be very fast with banning your soul into an object, else it will just find the next living thing (like Harry) to live off.
So, who was killed to create the Nagini Horcrux?