The Mechonis came into existence at the same time as the Bionis at the moment of their universe's creation. Both Titans lived in harmony until Bionis attacked Mechonis with the Monado to destroy the Machina. Mechonis tried to defend them but both Titans and their souls were put into a seemingly eternal sleep. During the battle, the Mechonis loose has its arm cut off by the Bionis, this arm fall into the ocean and becomes the Fallen Arm, which would be the place of hidding of the Machina after the battle.
Reawakening and Destruction
After Egil felt that Lady Meyneth betrayed him, he used Ether taken from the blood of the Bionis to force her body, the Mechonis, to move and attack the slumbering Bionis. Once Egil is defeated by the party, Dickson release Zanza from Shulk's body thus reawakening the Bionis. Egil use the Mechonis to try to stop him but fail, Lady Meyneth's soul is killed and Zanza return to Bionis to destroy and recreate the universe. Zanza attack the party which was trying to escape the collapsing Mechonis but Egil use the Mechonis body to protect them until Zanza destroy it completely.
"When the Mechon invaded, the lives of the peaceful inhabitants of Bionis were thrown into chaos. The ancient battle between the Bionis and the Mechonis had never truly ended at all, and for a time it seemed that the will of the Mechonis would succeed in wiping out all organic life. Until a hero appeared, wielding the legendary Monado..."
Mechonis can be considered female due to the fact that Meyneth, Mechonis' soul is female in the same way that Zanza, Bionis' soul is male.