The animal that is both a mammal and lays eggs is the monotreme, with the most famous example being the platypus. These unique creatures are known for their distinctive features, such as a duck-bill, webbed feet, and the ability to produce venom. Monotremes represent a small group within the mammalian classification and showcase fascinating evolutionary traits, such as egg-laying, which is more characteristic of reptiles and birds. Unlike other mammals that give live birth, monotremes maintain a unique reproductive strategy, making them intriguing subjects of study in the field of biology.