An herbivore was any animal that ate only plants, such as fruits and vegetables. An herbivore was unlike a vegetarian in that it did not make the conscious choice to not eat meat.
On Earth, the stegosaurus was a variety of dinosaur that was strictly an herbivore, of which fact Charles Tucker had to remind Malcolm Reed. (ENT: "Future Tense")
When captured by the Kzinti in 2269, the Chuft-Captain referred to Spock as an herbivore – an eater of "roots and leaves". Because of this classification, the Chuft-Captain refused to speak to Spock, and would only speak to Hikaru Sulu, because he at least ate meat.
The Kzinti Telepath found it extremely difficult to read Spock's mind, because he could see images of Spock eating vegetation. (TAS: "The Slaver Weapon")
The script of the Star Trek: Deep Space Nine episode "Homefront" described Jaresh-Inyo's people, the Grazerites, as having "evolved from herbivorous herd animals and as such loathe violence and confrontation."