Mibbiwocket is a giant tortoise, one of many farmed by gnomes for aid in battle. Mibbiwocket is currently the oldest tortoise held in captivity, and was presented to King Healthorg, a gnome king of the late Fourth Age, by the famous explorer Admiral Bake, shortly after the creature's birth.
Though little else is known about Mibbiwocket, it is known that he was the king's trusted mount, and was present at the Battle of Atarisundri, in which the gnomes managed to stop a bloody and vicious battle between themselves and the Thorobshuun goblin tribe. Mibbiwocket was used as a basemodel for Glouphrie who created an illusion with magic that made it appear as though the gnomes had an army of tortoises at their disposal, causing the goblin opposition to flee.
Mibbiwocket cannot be found in-game other than in one cutscene during The Eyes of Glouphrie, but is said to be in the care of the Gnome Royal Family. More information regarding the tortoise can be found in the Varrock Museum's Natural History Exhibit.