A Consumer that eats only plants is a _________?

A consumer which eats only plants is called an herbivore.

A consumer which eats only animals is called a carnivore.

A species which eats both plants and animals is called an omnivore.

With respect to humans, there is the possibility of some confusion. All humans are omnivores. Some people, however, are vegetarians. Vegetarians are those who choose to eat only plants. However, this does not mean vegetarians are herbivores. Vegetarians are omnivores because the human species is omnivorous.

In the same way, a cat which only has access to feed pellets composed of corn is still a carnivore, even though it may be eating a totally vegetarian diet. 

The definition of whether an animal is a herbivore, omnivore or carnivore is made on the species level, although individuals may consume different diets.