"If enough get together they can swallow you whole."
Doubts are normally see as grey worms with a mouth which are slightly transparent and jelly-like, and are first introduced in page 24 of Chapter 1: Rushes. If ignored, they don't do much damage, and fight each other instead, however too many Doubts would consume a person. They are encountered in the first part of the Hero's journey, and are also in the Plains of Hesitation.
Whilst they are normally worm-like in appearance, they are also able to morph, as they are shown to completely cover over the Hero in the Plains of Hesitation similarly to a blanket. It is also suggested that they weaken Amour, as the Hero's coat fades in color and becomes jelly-like when she is covered with Doubt. Their circular mouths with spirals of teeth are reminiscent of lamprey eels, suggesting a parasitic existence.