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 2Westwood re-added the Mammoth tank from the previous game to the Soviet arsenal as the 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 equipment. In addition, the models for the Mammoth appear as if the Red Alert 1 engine was still in use.