Why is the name aunt beast a good one for this alien creature?

Although Aunt Beast is all bug alien looking, she communicates (through thought) very gentle messages,

"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."

The creature, although looking like a beast, has the personality of a kind aunt.

A Wrinkle in Time