Here's why:
* Metaphor: A metaphor directly compares two unlike things without using words like "like" or "as." In this case, the person is being compared to a walrus.
* Simile: A simile compares two unlike things using "like" or "as." For example, "He was like a great walrus."
Since the sentence states "He was a great walrus," it implies a direct comparison without using any connecting words. This makes it a metaphor.