The other Horcruxes were more logical because they were non-living things. Non-living things never die biologically. Therefore, they are longer lasting compared to animals. Choosing a living thing like a snake is stupid because it is even questionable if a snake can outlive a human, not to mention being killed by other predators. Why did Voldemort choose a snake as a Horcrux in the first place?
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5Maybe he separated the snake's soul into 8 pieces so it would never die?– RichSMar 30, 2017 at 5:20
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1not sure of it's impact in the potterverse if any but it was a Snake took the object of immortality/rejuvenation that Gilgamesh sought (sometimes it's a plant, sometimes it's a potion)– Memor-XMar 30, 2017 at 5:50
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1Perhaps when the snake died of old age, it could still be useful as a horcrux (as a dummy)?– TimSparrowMar 30, 2017 at 10:07
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2It's not exactly specified but I think that beings which contain soul fragments are just as immortal as the person whose soul they host.– The Dark LordMar 30, 2017 at 10:24
1 Answer
We don't really know. We only have what Dumbledore told us:
"I don't think so," said Dumbledore. "I think I know what the sixth Horcrux is. I wonder what you will say when I confess that I have been curious for a while about the behavior of the snake, Nagini?"
"The snake?" said Harry, startled. "You can use animals as Horcruxes?"
"Well, it is inadvisable to do so," said Dumbledore, "because to confide a part of your soul to something that can think and move for itself is obviously a very risky business. However, if my calculations are correct, Voldemort was still at least one Horcrux short of his goal of six when he entered your parents' house with the intention of killing you.
"He seems to have reserved the process of making Horcruxes for particularly significant deaths. You would certainly have been that. He believed that in killing you, he was destroying the danger the prophecy had outlined. He believed he was making himself invincible. 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 Voldemort's 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, chapter 23 (emphasis mine)
So he made her the Horcrux because he still needed one, and he likes her.