Aunt Beast is one of the alien creatures that inhabits Ixchel, the planet the Murrys and Calvin land on after tessering off Camazotz.
They were the same dull gray color as the flowers. If they hadn't walked upright they would have seemed like animals. They moved directly toward the three human beings. They had four arms and far more than five fingers to each hand, and the fingers were not fingers, but long waving tentacles. They had heads, and they had faces. But where the faces of the creatures on Uriel had seemed far more than human faces, these seemed far less. Where the features would normally be there were several indentations, and in place of ears and hair were more tentacles. They were tall, Meg realized as they came closer, far taller than any man. They had no eyes. Just soft indentations. (10.78)
While the humans were not immediately frightened by the people of Camazotz, because of their familiar appearance, the alien look of the beasts produces an initial response of terror and even disgust. But while the people on Camazotz look like humans but act like robots, the beasts look far stranger but share with their human visitors the individuality that the Camazotzians lack. The beasts do, however, also share the Prime Coordinator's abilities in the mind-reading department, as Aunt Beast demonstrates when she probes Meg's thoughts in order to name herself.
While Meg thought, the beast murmured to her gently. "No, mother is a special, a one-name; and a father you have here. Not just friend, nor teacher, nor brother, nor sister. What is acquaintance? What a funny, hard word. Aunt. Maybe. Yes, perhaps that will do. And you think of such odd words about me. Thing, and monster! Monster, what a horrid sort of word. I really do not think I am a monster. Beast. That will do. Aunt Beast." (11.69)
Even though Aunt Beast shares IT's telepathic powers, her use of them is very different from the brain dude's. While IT focuses on conquering other minds, Aunt Beast uses her power in service of communication, and making an affectionate connection between herself and Meg.
The theme of communication is key to Aunt Beast's role in the story. It turns out that it's not her lack of mouth that is the main barrier to shooting the breeze with the humans, but her lack of eyes. As Meg realizes first when she tries to explain light, and then when she attempts to conjure Mrs. Whatsit, her very language is shaped by how she sees things, which makes talking to a being that has no conception of sight difficult to say the least. Aunt Beast's lesson for Meg is not, however, of the limitations of blindness, but rather of the limitations of sight:
But she realized now that here on this planet there was no need for color, that the grays and browns merging into each other were not what the beasts knew, and that what she, herself, saw was only the smallest fraction of what the planet was really like. It was she who was limited by her senses, not the blind beasts, for they must have senses of which she could not even dream. (11.74)
Meg's relationship with Aunt Beast drives home once and for all the message that appearances are deceiving, or at least insufficient; the things that really matter are beyond the reach of sight.