Reptilicus Metallicus is a large robotic snake or worm creature that lives in a metallic burrow in the ceiling of Moon 13. The "creature" can be deployed by inserting a large key into the back wall of Moon 13, and turning it once clockwise.
Reptilicus Metallicus was first discovered by Max accidentally while writing in his diary. The creature considered Max his master, but the moment was interrupted accidentally by Jonah Heston, prompting Max to relock the monster away and drop The Tube onto Jonah.
During Kinga Forrester's wedding to Jonah, Max, who had become insanely jealous of Jonah essentially taking Kinga away from him, unleashed Reptilicus Metallicus before the wedding-goers as well as the bride and reluctant groom and proceeded to eat Jonah before disappearing. 
It is currently unknown what happened to Reptilicus Metallicus after Jonah escaped.