Dominated? I just checked my cat's food, and the first ingredient is chicken and the second is chicken by product, then corn and the list of supplements. The dominant ingredient is chicken.
Are you sure there's a regulation regarding listed ingredient order for pet food? And regardless, I bet manufacturers can get away with even more ingredient list manipulation there than with human food—I might be wrong though.
It depends on the type and quality. If you purchase a can, then I would hope that it is mostly animal based ingredients. If it is dry food then it will be mostly cereals.