Here's what happens:
* Mrs. Whatsit's Form: She is described as a "large, rather formidable-looking woman" with a "huge, floppy hat," but she can shift her appearance at will.
* The "Creature" Form: In the book, she does shift her form when Meg, Charles Wallace, and Calvin are traveling through time and space. She takes on a kind of "creature" form to help them navigate through a hostile environment. This form is described as being "large and dark and shadowy" and resembling a giant, winged, bat-like creature.
* Purpose: Mrs. Whatsit takes this form to protect them and help them move through the perilous environment.
It's important to note that she doesn't really *turn into* a creature, but rather shifts her form to a more appropriate one for their current needs.