Your challenge--by altering evolution itself, human settlement, or the rest of the fauna of the hemisphere, create cougars/mountain lions, Puma concolor, that exist and hunt in a group or pack like African lion prides. The idea is not totally ASB to me--mountain lion cubs that were raised and kept together often maintain a lifelong bond, so there are some social instincts in the species, however minimal.
Possible PoDs:
1--Greater survival of huge megafauna to both provide more food mass, and make it more difficult for any one lion to bring down its meal by itself
2--Flatter, less densely vegetated terrain across the Americas to make it harder for the cats to isolate themselves
3--Much later human settlement to reduce hunting pressure so that the "hermits" are not more likely to survive and reproduce
DaleCoz, I think you'd have fun with this one...