Smellies are creatures who only appear in the episode "The Thing."
They have light gray colored hair and have a leaf like object on their heads.
SpongeBob and Patrick mistook Squidward for one of these because he got stuck in cement, and the S.W.A.T. attempted to capture him. In the end, they got him out of the zoo and carried him into a Kelp Forest, where Kelpy G is playing his clarinet, and it appears the Smellies enjoy his music. When the cement cracks off of Squidward, they look at him strangely.