During the mission "X Marks the Spot", you'll be swallowed by the Leviathan. Inside of the Leviathan, you'll be able to find some loot and vending machines but you'll eventually drop into a large area where you'll find Captain Scarlett riding on her pet, Roscoe.
Roscoe is very similar to a Rakk Nest type monster encountered in the first Borderlands. The battle is much easier early on than it is later because over time, he'll create more and more Rakk that will harrass you. He will also spit acid blobs at you as his own direct form of attack. He is a little tougher than his predecessor from the previous game because the weakest point on his body are his eyes but they are covered up with blinders so it is much harder to find them.
Look for the black patches that cover his eyes and aim for those. You'll deal much more damage. Try to keep moving but watch out for the pools of acid on the ground. Try to leave as many Rakk in the air as you can without allowing them to become to much for your shields to handle because if you get downed, they will definitely be the easiest way to get back on your feet. Simply shooting into a crowd of Rakk will likely kill one of them.