The Skeleton Mom appears in "City of Thieves." She screams when Big Guy steals her baby. According to Penny, "Everyone steals from each other" in the city, so it is possible that she stole the baby beforehand. This is supported by the fact that the baby is a different color than her.
Skeleton Mom is a skeleton who wears a purplish-brown Victorian-style dress and has boots. Her bones are white, unlike her baby's gold ones.