Milkshake Gremlins are paranormal creatures that sneak into milkshakes to duplicate. Only one appears in the episode "Ghoul Fools."
The Milkshake Gremlin is a very small light green creature that appears to have a unibrow and two small light yellow horns. It also has even smaller yellow horns in between the larger ones, which go across its head. The Milkshake Gremlins have a habit of hiding in milkshakes.
When SpongeBob and Patrick hear a disturbing noise, it actually turns out to be Sandy's new device she is using to detect Milkshake Gremlins.
Near the end of the episode, Patrick hands Sandy a milkshake and she finally catches a Gremlin from it.