Some of the animals commonly mistaken for cougars: deer, lynx, coyotes, fishers, dogs, and house cats. Big ones.
Coyotes, I can understand. Lynx? Sure, why not. Deer? Okay, but only if the spotter has never seen a cat of any kind before. House cats? Seriously? You’ve got to be seriously spooked if you’re confusing your neighbour’s kitty for a cougar. Then again, if you have any giant mice hanging around your house, you’d probably want a giant kitty for protection.