Green satchels are a drop from unicows, which can be created in the basement of the Tower of Life after completing the Tower of Life quest. Unicows can only be created during the Creature Creation Activity. It holds a banana, a cake and triangle sandwich. It will also change the beckon emote.
When full, the food contained heals just a little less than a monkfish. It takes multiple bites to consume the food and is potentially more expensive.