Galioct

The Galioct is a mutant lizard. It was formed when a growth spell went haywire. The spell was supposed to increase its size from 3 centimetres to 10, but it made it 100 metres tall!

Rampages
Galioct went on a rampage through Gielinor and crushed buildings and attacked people. The cities attacked were: Varrock, Lumbridge, Seers' Village, Taverley, and Falador. Almost all of these cities were 100% destroyed.

Varrock Rampage
The Varrock Rampage was Galioct's most infamous rampage, Galioct destroyed 99% of the city, the only buildings still standing, albeit partly, were: the Grand Exchange, the cage with a bear, and part of Varrock's East Bank. The rampage lasted almost a month before citizens lured Galioct out of the city.

Death
Galioct's death was caused when mages loaded up the Air Destroyer and fired all its power at Galioct, vaporizing him into nothingness.

Mark on Gielinor
Even after his death, Galioct still has a mark on Gielinor, from footprints to scorch marks on buildings, anyone who survived the rampages will never forget Galioct.

Trivia

 * Galioct was the monster chosen for the film Galioct the Great
 * Galioct was inspired by Godzilla, this users favourite kaiju.
 * This is my entry in the Gielinor's Beastiary contest.