Mammals that usually eat only plants are called herbivores.
Omnivores eat both plants and animals.
Insectivores eat insects. Some herbivorous mammals are very choosy. The koala bear, for instance, eats only the leaves of 12 kinds of eucalyptus. Others will eat almost any plant if they have to. Herbivorous mammals either graze, eating mainly grasses, or browse, eating mainly leaves, bark and buds of bushes and trees.