What does Mrs. Whatsit change into in A Wrinkle in Time?

Mrs. Whatsit turns into a Centaur-like creature more beautiful than Meg has ever seen. 

Mrs. Which tells Mrs. Whatsit to "Sshoww themm," and Mrs. Whatsit asks if she should “change.”  Mrs. Which says yes, and Mrs. Whatsit transforms before their eyes into “a creature more beautiful than any Meg had even imagined.” 

She was a marble white body with powerful flanks, something like a horse but at the same time completely unlike a horse for from the magnificently modeled back sprang a nobly formed torso, arms, and a head resembling a man's, but a man with a perfection of dignity and virtue ... (Ch. 4) 

Mrs. Whatsit sprouts “wings made of rainbows,” which Meg describes as poetry.  Calvin falls to his knees at the sight, but Mrs. Whatsit tells him, “Never to me.”  Mrs. Which commands Mrs. Whatsit to carry the children on her back.  Calvin asks what they should call her, and she tells them she is still Mrs. Whatsit and they can’t change what they call her every time she changes shape. 

Then Mrs. Whatsit takes them on a “serenely smooth” flight through the landscape of the planet Uriel.  They see plains, rock monuments, and a beautiful garden.  They see other creatures that look like Mrs. Whatsit’s new shape.  They are singing, but the children do not understand the words.  Charles Wallace does, but he says he doesn’t know enough to translate.  Mrs. Whatsit sings the song for them, which celebrates the landscape and the glory of the Lord. 

They move up into the clouds, where they use flowers to breathe.  Mrs. Whatsit tells them to look toward the darkness.  Then they see the Black Thing. 

Meg's eyes ached from the strain of looking and seeing nothing. Then, above the clouds which encircled the mountain, she seemed to see a shadow, a faint thing of darkness so far off that she was scarcely sure she was really seeing it. (Ch. 4) 

The Black Thing is terrible.  Calvin asks Mrs. Whatsit to make it go away saying, “It's evil."  They return, and Meg asks if that is what her father is fighting.  She is told that he is behind the darkness, so they can’t see him.