Can anyone identify this slug-like creature? Is it harmful?
Note: this photograph was taken in Pennsylvania, U.S.
Appears to be some leopard slugs (Limax maximus). Can't be 100% sure since you didn't include the area the slugs were found but for the UK or North America it'd be right. They mostly eat rotten vegetation and potentially other slugs. It is not considered a pest and some consider them useful since they do eat some pest slug species.