A couple new models news

Now I present you Rise of Omnius Mammoth MK III and Mammoth MK IV.

The Mammoth MK. III battle walker is the successor of the old experimental mech known as Mammoth MK. II. It's main difference with the original walker, is the addition of a deploy mode, which enables the use of a turret, since the original design couldn't have one while mobile, as it could fall on it's side. This mech is, however still able to fire it's weapons on the move, but they must remain on a fixed position.

Actually there are few MK. IIIs still in serviced. As it happened to their predecessor, they're being slowly replaced by more modern designs, based on it.
Like the Leviathan, which replaced it, as the heaviest experimental unit, this was done because the greater mobility and flexibility of it. Which allowed Leviathans to be transported to every point in the world, by air and space aboard dropships.
However the Leviathan is not a part of the Mammoth design line.

But the MK. III was not the end of the Mammoth Walkers. A new model based on it, was recently introduced, equipped with sonic shock artillery cannons, similar than those found on the Leviathan. Because of this, the Mammoth MK. IV, has replaced the Juggernaut, as the Global Defense Initiative, mobile artillery unit.

However smaller, lighter and less armed than its predecessor. The Mammoth MK. IV is still a fearsome enemy, or a great ally. It has a longer range (while deployed), similar to the one that Juggernauts had; it is also tougher, more accurate, and because of this, stronger than it, but at the same time, it is also more expensive than Juggernauts. A necessary sacrifice, in a world where the GDI cannot afford quantity.