Describe Mrs. Whatsit's true form

From the text:

"Now, don't be frightened, loves," Mrs. Whatsit said. Her plump little body began to shimmer, to quiver, to shift. The wild colors of her clothes became muted, whitened. The pudding-bag shape stretched, lengthened, merged. And suddenly before the children was a creature more beautiful than any Meg had even imagined, and the beauty lay in far more than the outward description. Outwardly Mrs. Whatsit was surely no longer a Mrs. Whatsit. 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, an exaltation of joy such as Meg had never before seen. No, she thought, it's not like a Greek centaur. Not in the least. From the shoulders slowly a pair of wings unfolded, wings made of rainbows, of light upon water, of poetry."