The Mamut is a scrapped enemy in Serious Sam 1.
The Mamut had two attacks; gore (or fire bullets if mamutmen are riding it) and stomp the ground. There were meant to be two Mamut skins; a skin with brown fur ('Summer') for most of the maps it appeared in, and a white one ('Winter') for the Ice Planet.
One thing of note is that another scrapped enemy, the Mamutman, was capable of riding the Mamut. The Mamutman would automatically be ejected when the Mamut lost 600 health points.
The Mamut was based on the Mammoth, clearly shown by its appearance and by the fact that the Croatian word for 'mammoth' is 'mamut'.
Mamuts are one of the few scrapped enemies that appeared in the pre-release media, as seen below.
The Mamut appears as a functioning enemy in Serious Sam Revolution and the upcoming Serious Sam Origins.