Once the bone mech was created, Lady Bone Demon used the mech to attack MK. Although MK was able to destroy the mech in its spider form, Lady Bone Demon rebuild it into the Bone Scorpion after MK lost some of his powers.
After MK destroyed the mech's scorpion form by creating the Monkey King Warrior Mech, Lady Bone Demon changed into her true form and transformed the bone mech into the Bone Demon.
Monkie Kid and Mei must defeat the Bone Demon three times. But this shape-shifter is devilishly tough to beat, so they'll have to use all their skills… a mech, Cloud Board and jetpack might help too!
The Bone Spider was the first form of the bone mech. After the bone mech was created, Lady Bone Demon used the mech in its Bone Spider form to attack MK, but was eventually destroyed when MK enlarged himself.
The Bone Scorpion was the second form of the bone mech. After MK lost some of his powers, Lady Bone Demon rebuilt the bone mech into the Bone Scorpion to continue her battle with MK.
The Bone Demon is the third and final form of the bone mech. After the Bone Scorpion gotten destroyed by MK, the Lady Bone Demon changed into her true form and rebuild the bone mech into the Bone Demon.
The bone mech's three forms are a reference to Lady Bone Demon's disguises in chapter 27 of Journey to the West, where Lady Bone Demon took on a different form everytime Monkey King seemingly killed her.