These are needed to carry foraging items back to the village, such as kelp from the beach, or herbs, fruit and nuts from the valleys, even kindling wood.
The panniers could be made out of a combination of cloth, or willow and/or reeds.
These could decay slowly with use and would need reeds to improve using thatching (weaving) skill.
They could be an adaptation of the existing saddlebags item.
The donkeys 'carry more' trait (or similar) could enable heavier loads.