Why didn't Voldemort turn the Basilisk into a Horcrux?

Question

Ok, Voldemort (Tom Marvolo Riddle) created 7 Horcrux's including his pet snake Nagini... Well why didn't he just turn the Basilisk into a Horcrux instead or also?

Answer 1

Dumbledore tells Harry that it is very inadvisable to turn a living animal into a Horcrux, considering that they can think and act for themselves. Voldemort apparently thought the same, and turned the diary into a Horcrux instead. Every Horcrux requires a murder: after using Myrtle's to create the diary, he would have had to blow his cover and murder a second student to make the basilisk one too.

But Voldemort was one Horcrux short when he went to attack Harry (intending to use Harry's death to make his sixth). When he was finally able to wield a wand and create the seventh piece of his soul, he was very limited in his options.

But Dumbledore suggest an even simpler reason why he chose Nagini instead of the basilisk: he was kind of fond of her.

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 mys-tique; 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."

So the answer to "Why did Voldemort make Nagini a Horcrux instead of a giant murder snake," might be no easier to answer than "Why do people buy cats instead of dogs who can defend them?" Different strokes for different folks.

Answer 2

There's no clear indication, but there are several possible reasons

He didn't think of it

I suspect that it simply did not occur to him to turn the basilisk into a Horcrux until much later on. For example, it took him a while to conceive of making Nagini a Horcrux. Perhaps no one had ever turned a living thing into a Horcrux previously.

“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.

(Harry Potter and the Half-Blood Prince)

If he had not conceived of making the Basilisk a Horcrux soon after he graduated, he simply would have missed the opportunity, since by then he would not have been allowed to set foot in Hogwarts. As we see, he did place the Diadem Horcrux there, but after that he would not have been welcome.

He couldn't

Creating a Horcrux requires a number of foul rituals. These rituals, crucially, may take time. Voldemort might have needed to spend days or weeks in the chamber in order to turn the Basilisk into a Horcrux. Dumbledore might have noticed him show up weeks later looking even worse, and conclude what he had been doing. He might even have learned the location of the Chamber, and thus both the Basilisk and Voldemort's Horcrux. Certainly Dumbledore would have noted that he had not returned to his friends, and deduced he was still in the castle.

“Then if I were to go to the Hog’s Head tonight, I would not find a group of them — Nott, Rosier, Mulciber, Dolohov — awaiting your return? Devoted friends indeed, to travel this far with you on a snowy night, merely to wish you luck as you attempted to secure a teaching post.”

(Harry Potter and the Half-Blood Prince)

All of this was too much for Voldemort to risk.

What's more, perhaps he needed to commit the murder within close proximity of the Horcrux, or within a certain time period of making the Horcrux. Especially given that Voldemort liked to use significant deaths to make Horcruxes, this would have made the process of turning the basilisk into a Horcrux difficult.