There are many species of predator that could act as such a challenge, wild animals that could easily catch, kill, and consume an incautious human. A lion, for instance, could deal almost certain fatality to a human, if the animal so desired him as a meal or perceived him as a threat. In fact, a human is no match for a lion's stealth, speed, and power. A large serpent such as an Anaconda could also easily swallow and digest even a large human if presented one. However, its inherent sluggishness and lethargy render it unlikely to manage such a feat, assuming the target is mobile and comprising the five senses. A creature somewhere in between the prowess of a lion and the sluggishness of an Anaconda may well be a good match for a human.
A bear, perhaps, stands as the most appropriate middle ground. Bears can be fierce but often cumbersome, lethal but not exceedingly stealthy, and a human may have a fair chance against a bear, if they are paying heed to their surroundings. Certainly, an unwary human in a famished bear's habitat stands little chance. However, humans have the advantage of intellect, and in a situation where the human is in familiar territory and is aware of the possibility of an attack, he has a fair chance of evading an encounter.
Therefore, and for the betterment of mankind, I propose we introduce bears into society. This will serve a grand purpose, to reintroduce a primal element of Natural Selection into human populations. For without Natural Selection, the idiots continue to reproduce, and our species cannot improve.
This, of course, must be carried out systematically:
Because the United States is the most modern, industrialized nation in the world, its city centers would be the first to incorporate bears. If the goals of this project were being met, or at least if its successes outweighed its failures, bear incorporation could begin elsewhere in the developed world.
It would become legal in all city centers and areas of the US in which bears per capita equaled less than the national average for enough animals to be released to reach that limit. Release of bears would be strictly limited to licensed, federal wildlife officials who were specifically appointed in the management of this project. Specific, mapped points of introduction would be carefully coordinated taking into consideration predicted roaming patterns and habitat usage of each introduced animal, as well as human population densities. This would ensure even dispersion of bears among groups of humans.
Animals to be released would be born of wild bear mothers in captivity, and reared under such provisions until they were able to survive on their own. Thus, the bears would be juveniles at release, old enough to be self-sustaining but young enough to be non-threatening on introduction. This would allow humans in the area to learn the bear's habits before the animal became lethal. Humans who chose to be observant at the beginning would have less chance of an encounter. Only female animals would be released, as to avoid breeding and territorial disputes between individuals.
By growing up in city centers, the bears would learn to live in human areas and to tolerate human presence, which would habituate the animals to human activities and illuminate humans as prey items. Furthermore, naturally occurring bear populations would help keep introduced bears within their desired ranges.
Attacking or killing a bear in situations other than self defense would be punishable by law. All instances of fatality of both human and bear would be thoroughly reviewed by a panel of legal administrators, specially trained forensic examiners, as well as local law enforcement in order to determine circumstances and legality of death.
As to ensure all humans' participation in this project at all times, there would be a strict "no-limits" clause in regard to where the animals could roam. If a bear wandered into a Wal-Mart, college fraternity, or the Jersey Shore beach house, no measures would take place to remove it until it was killed during an altercation or it decided to exit.
Crucial to this project would be to minimize casualties that were not a product of the overall goal. After all, the objectives would not be a drastic reduction in human population size nor to endure copious innocent lives lost. By releasing only enough animals to reach natural, average bear densities per capita, humans would still grossly outnumber bears, and each human's chance of an encounter would remain relatively small. However, the possibility is what would change behavior. One could not completely engross themselves in their fast food burger, bury their head in their mobile device in disregard to the rest of the world, or act otherwise idiotically, because they might get attacked and devoured by a bear. Those that persisted in such a lifestyle would become the individuals most likely to be selected out of the population.
This survival-of-the-fittest schema would drastically increase the overall quality of mankind by producing a human species that was more adept, involved, and aware of their environment. The knowledge that, at every point in the country, somewhere within several miles, there is almost certainly a man-eating bear, would force humans to adapt, to improve. It would force humans to evolve.