Are mammals the only animals that yawn?

No. If you mean opening the mouth very wide and breathing deeply then some fish, birds and even snakes yawn. Baboons, guinea pigs and Siamese fighting fish yawn to show aggression. Some penguins yawn during courtship rituals (but presumably not because they’re bored). Some snake’s yawn after a good meal. We don’t yet understand all the many functions of yawning.