Some explanation here. Omnivore is a scifi novel by Piers Anthony. It describes, through the eyes of a small team of humans, a planet whose life has evolved from fungi. The manta lifeform is  the top predator --here protecting one of the earthmen, Veg, from a carnivore. The novel is heavily didactic and the omnivore is actually Man, i.e., the most destructive of all species. More on the subject here.