"A huge herd of elk (or some sort of reindeer type animal)....."
Caribou. Specifically, barren ground caribou, the bigger and better cousins of reindeer. They travel in huge herds, always moving, and to watch a couple of hundred flowing across the landscape is an amazing experience.
As I understand it, the musk ox were reintroduced recently. Either you're lucky to have seen them or there are far more than there used to be. I've seen them in Greenland, and when you see how they gather up to defend themselves it's obvious why they got wiped out so easily by hunters with guns.
I camped on the North Slope as well, just off the road outside of Deadhorse. It was August, and it snowed overnight--not enough to get seriously in the way, but enough to remind us who was boss. We saw quite a few bears up there, so be wary and smart about your camping.