The following is based on Red Alert 2 cut content and has not been confirmed by canon sources.
During the development of Command & Conquer: Red Alert 2, the Mammoth tank from the previous game was originally intended to be part of the Soviet arsenal as its most powerful vehicle, which again used two cannons and AA/AP missiles. Later in the development however, Westwood decided to introduce the Apocalypse tank to replace it. All what remains of the Mammoth tank now are just screenshots from the RA2 beta.
It is possible that rather than completely scrapping the Mammoth, it was simply given an complete graphic overhaul; as the Apocalypse and Mammoth are nearly identical in appearance and weaponry. In addition, the models for the Mammoth appear as if the Tiberian Sun engine was still in use.