No.
Most mammals are placental...marsupial mammals and monotremes are not placental.
Dolphins are placental mammals.
Elephants are placental mammals.
All bears are placental mammals
A cow is a placental mammal.
No. Seals are placental mammals.
No. Armadillos are placental mammals.
No. Camels are placental mammals.
No. Hyenas are not marsupials. They are placental mammals.
Bobcats are not marsupials. They are placental mammals, while marsupials are pouched mammals.
Placental mammals reproduce externally. (not in pouches)
Seals are placental mammals, as the young complete their development within the mother's uterus, attached to a placenta. They do not have a pouch like most marsupials, and they do not lay eggs like monotremes.