No. A myna is a bird.
A crocodile is a reptile, not a mammal.
Marine mammals are divided between the orders Cetacea, Sirenia, and Carnivora. Whales, dolphins, and porpoises are in the order Cetacea. Manatees and dugongs are in the order Sirenia. Seals, sea...
They mean the same thing it is just that mammals is the plural form of mammal.
MAMMAL