They are green K-9 worms. They have silver collars with spikes on them and are very vicious. They look like other worms in the series, such as JK.
When the police sergeant calls for backup to get rid of the Slide Whistling from SpongeBob, Patrick and Squidward, the K-9 Unit shows up in a SWAT van. But Squidward then plays his slide whistle and the worms start acting up. They then attack the K-9 Handler, who yells at them. Beaten up, the handler says, "I was supposed to retire this week!"