In practice, it's an evil humanoid press-ganged into a servant. The group in my current campaign has "earned" the services of a goblin named Bootlicker who the other goblins all hated and agreed to give him to the party. In the previous campaign, they captured a pair of pirates named Ginger and Man-Bun and convinced them to sign on to the PCs' ship as the first NPC crew members. In the campaign before that, a kobold named Squeaker and his buddies were always running errands for the PCs; they would often show up and offer to help carry loot in their "bags of holding" (just regular sacks, but the kobolds were holding them).