A huge endangered trance worm that Cleveland Booker steals back from another courier in 3188 and returns to a sanctuary world, swallowing several of his pursuers along the way.
Molly appears in That Hope Is You, Part 1. She is the temperature-sensitive cargo in the hold of Book's ship, the thing every courier and enforcer on Hima is chasing him for.
Burnham assumes he is an ordinary thief until the pursuers force open the hold. Molly emerges, swallows some of them and briefly swallows Burnham as well, spitting her out when Book asks. He can speak to her through the empathic bond his people share with living things.
Book then jumps to a sanctuary planet and releases her among others of her kind, explaining that trance worms are close to extinction now that the Federation is no longer able to protect them.