Among those animals, what are mammals? platypus, Greenland shark, puffer fish, rattlesnake, orca, kangaroo, ostrich

From above, those are mammals: platypus, orca, kangaroo. Greenland shark and puffer fish are fishes, not mammals. Rattlesnake is a reptile. Ostrich is a bird. Note though orca lives in the sea and looks like big fishes, e.g. sharks, orca is in fact a mammal.