The term 'Hamelin Pest' refers to the story of the Pied Piper of Hamelin, a folktale originating in Germany. In the story, a RAT-catcher is hired by the town of Hamelin to rid the town of its RAT infestation. The RAT-catcher succeeds in doing so by playing a magical pipe, which causes all the rats to follow him out of town. The crossword solution for this term is 'RAT', which is a three-letter word that accurately describes the creatures that were plaguing the town of Hamelin.