where does the term come from?
Herbivore is the anglicized form of a modern Latin coinage, "herbivora," cited in Charles Lyell's 1830 "Principles of Geology."
Herbivore
Herbivore is the anglicized form of a modern Latin coinage, "herbivora," cited in Charles Lyell's 1830 "Principles of Geology." 
how do some chemicals in plants protect them from their enemies Through allelochemicals  that influence the behavior, growth, or survival of herbivores. name some animals/species that fall under the last-mentioned category. White-tailed deer , elephants, zebras, and giraffes. how would this category be defined? A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet.
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Herbivore [SEP] Etymology
Herbivore is the anglicized form of a modern Latin coinage, "herbivora," cited in Charles Lyell's 1830 "Principles of Geology." Richard Owen employed the anglicized term in an 1854 work on fossil teeth and skeletons. "Herbivora" is derived from the Latin "herba" meaning a small plant or herb, and "vora," from "vorare," to eat or devour.