name some animals/species that fall under the last-mentioned category.
White-tailed deer , elephants, zebras, and giraffes.
Herbivore
Herbivory can have impacts on both economics and ecology. For example, environmental degradation from white-tailed deer ("Odocoileus virginianus") in the US alone has the potential to both change vegetative communities through over-browsing and cost forest restoration projects upwards of $750 million annually. Agricultural crop damage by the same species totals approximately $100 million every year. Insect crop damages also contribute largely to annual crop losses in the U.S.
Herbivores affect economics through the revenue generated by hunting and ecotourism. For example, the hunting of herbivorous game species such as white-tailed deer, cottontail rabbits, antelope, and elk in the U.S. contributes greatly to the billion-dollar annually, hunting industry. Ecotourism is a major source of revenue, particularly in Africa, where many large mammalian herbivores such as elephants, zebras, and giraffes help to bring in the equivalent of millions of US dollars to various nations annually.
how do some chemicals in plants protect them from their enemies Through allelochemicals  that influence the behavior, growth, or survival of herbivores.
wiki:11588868
Herbivore [SEP] Impacts
Herbivorous fish and marine animals are indispensable parts of the coral reef ecosystem. Since algae and seaweeds grow much faster than corals, they can occupy spaces where corals could have settled. They can outgrow and thus outcompete corals on bare surfaces. In the absence of plant-eating fish, seaweeds deprive corals of sunlight. Herbivory can have impacts on both economics and ecology. For example, environmental degradation from white-tailed deer ("Odocoileus virginianus") in the US alone has the potential to both change vegetative communities through over-browsing and cost forest restoration projects upwards of $750 million annually. Agricultural crop damage by the same species totals approximately $100 million every year.