Piggy was the name given by Molly O'Brien to her stuffed toy targ. Molly kept Piggy with her when she stayed at Deep Space 9, and was often seen clutching onto Piggy in her quarters.
In 2371, Miles O'Brien suggested that Molly go into her bedroom and play with Piggy while he attempted to cheer up Mommy.(DS9: "Fascination")
Piggy may have been a toy version of Toby the targ, which existed in the same era.