Find Scratch just outside the Blighted Village in Act 1. Interact with him and convince him to meet you at your camp. After taking multiple long rests at camp and petting Scratch, he will have an exclamation mark over his head. When you interact with him again, there will be a ball in his mouth that you can remove. Then, go into your inventory and select to Throw the ball near scratch. After he picks up the ball and brings it back to you, the Fetch Quest achievement will pop.
Scratch will occasionally find a random object after completing a long rest. Check to see if he has anything for you.