How come Nagini had magical powers?

Question

Nagini was able to transform into Bathilda Bagshot and was waiting for Harry to come to Godric's Hollow. Did the snake have magical powers, or did Voldemort transform her into Bathilda? If it's the latter, was it possible for a normal wizard to give special powers to creatures like snakes, or is it just because Voldemort was able to speak Parseltongue?

Answer 1

I think that when Lord Voldemort has created a horcrux, he gave it the power to project solid moving images of people, and to magically convince targets that those images are real. We've seen Riddle's diary and Slytherin's locket perform the same magic, and this is what Nagini has done as well.

In the Chamber of Secrets, we meet the figure of the young Riddle. He is solid enough to pickpocket Harry's wand. When Ginny wakes up, she says she remembers Riddle “coming out of the diary”.

Now consider what Dumbledore says about that occurrence in the Half-Blood Prince chapter 23 (Horcruxes). He claims that he took these events as “certain proof that Voldemort has split his soul”. Mind you, however, that the clue Dumbledore mentions not the image of Riddle the diary has created, but how the diary was “starting to act and think for itself”. This could mean either of two things: either Dumbledore believes that a horcrux would likely also have the power to project images, or that projecting solid images is an easy kind of magic compared to all the other charms Lord Voldemort would put to the Horcruxes.

In the Deathly Hallows chapter 19 (The Silver Doe), we see the distorted images of Harry and Hermione emerge from the locket. It's hard to tell how belivable the illusions appeared for Ron, because he doesn't talk about it, but they certainly had great effect on him.

In both of these cases, the illusions might have been lifelike, but they didn't fool at least Harry to believe them real. However, the circumstances are different in the case of Nagini imitating Bathilda Bagshot. In the case of the diary, Harry knew that the real Riddle is much older than the image is, which is why he immediately asks the young Riddle whether he is a ghost. For the locket, Harry has actually seen the images come out of the locket, and knew where the real Harry and Hermione were. In Godric's Hollow however, Harry and Hermione did expect to find Bathilda in the town, so they didn't have a reason to doubt the illusion. In this last case, Lord Voldemort likely gave detailed instructions to Nagini recently, armed with current knowledge about what would fool Harry.

There's one disparity my theory doesn't explain though. The image of Riddle could speak plain English, and so did the image of Harry and Ron. Why did, then, the image of Bathilda not speak except in Parseltongue?

Update: on the illusion the diary has created, see If diary Tom Riddle had succeeded, would there have been two Voldemorts? , If the Diary Tom Riddle Had Succeeded, Would He Have Been a Horcrux? and Why could Voldemort's memory from the diary horcrux take physical form?

Update 2018-09-26: Apparently the Fantastic Beasts film series will elaborate on Nagini and his imitating Bathilda Bagshot, with an explanation possibly contradicting the above. The recent question “Was Nagini the only snake which could become human? / Recently launched trailer of Fantastic Beast 2: Crimes of Grindelwald has proved that being Animagus (I don't have proper word) was Nagini's inherent quality, not something assisted by Voldemort when she had became Bathilda Bagshot” called my attention to this. The answer “Not an Animagus. A Maledictus. Big difference. - J. K. Rowling on Twitter” contains words directly from J. K. Rowling.

Answer 2

Nagini didn’t transform to Bathilda - she was in Bathilda’s corpse.

Nagini hadn’t been Transfigured into Bathilda, she was inside Bathilda’s dead body. That’s why Harry thought either Bathilda or her house smelled bad.

“She smelled bad, or perhaps it was her house: Harry wrinkled his nose as they sidled past her and pulled off the Cloak.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)}}

Later, Harry smelled something that he thought was like spoiled meat - because Bathilda was actually a corpse.

“Thick dust crunched beneath their feet and Harry’s nose detected, underneath the dank and mildewed smell, something worse, like meat gone bad. He wondered when was the last time anyone had been inside Bathilda’s house to check whether she was coping.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)}}

Nagini doesn’t seem to have been able to do magic, and while in Bathilda’s corpse, was fairly clumsy and slow-moving.

“She seemed to have forgotten that she could do magic too, for she lit the candles clumsily by hand, her trailing lace cuff in constant danger of catching fire.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)}}

Finally, when Nagini was ready to attack Harry, she burst out of Bathilda’s corpse, leaving the corpse to collapse and fall away.

“And in the instant that he looked away, his eyes raking the tangled mess for a sword hilt, a ruby, she moved weirdly: he saw it out of the corner of his eye; panic made him turn and horror paralysed him as he saw the old body collapsing and the great snake pouring from the place where her neck had been.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)}}

It wasn’t a transformation or illusion - Nagini was using the corpse itself as a disguise.