Bring a Shattertusk Mammoth to Zootfizzle at Dorian's Outpost.
Before you go off challenging Broodmother Slivina, Dorian thought it might be a good idea for you and Hemet to coordinate your efforts by riding into battle on the back of a mammoth.
The area around the camp is full of them. Just take this bridle and slip it over the head of a shattertusk mammoth. Be careful of the bulls, though. They won't tolerate the harness at all. Bring the mammoth back here when you've found a suitable one.