Choose the word which is different from the rest.

The answer is D. Dinosaur.

A mermaid is a mythical creature with the head and upper body of a woman and the tail of a fish. A sphinx is a mythical creature with the body of a lion and the head of a human. A unicorn is a mythical creature with the body of a horse and a single horn on its forehead. A dinosaur is a prehistoric reptile that became extinct about 66 million years ago.

Mermaids, sphinxes, and unicorns are all mythical creatures that are still believed in by some people. Dinosaurs, on the other hand, are extinct creatures that are known to have existed because of fossil evidence.