Due to the nature of tabletop RPGs, a campaign will often start with a random group of strangers. Here are some ways to get them adventuring together, without resorting to the old Tavern trope.
Most large mammals, such as elephants, rhinos, and their modern ilk are ill-suited for domestication, but that doesnt exclude their use as engines of war. Rather than raise and train the Mammoth from birth, Mammoth Handlers instead capture the wild Mammoths at a young to juvenile age and train them for war. These animals are draped in armor and are unleashed as living war machines against the foes of the Mammoth Tribe.