The Grumblebug is a Petpetpet (a pet for your Petpet).
This little bug makes a grumbling sound. Many Petpets get scolded when they have a Grumblebug.
You can buy this Petpetpet from the Shop Wizard for around 40,000NP.
There is one item associated with this Petpetpet; "Grumblebug Water Bowl".