Camels — Camels are known for their stubborn and crabbish personalities, but consider this: Camels have a gestation period of 13-14 months or around 410 days. Other camelids, such as llamas, also have a long gestation period — about 330 days.
Walruses — Walruses have the longest gestation period of all pinnipeds (a group of mammals that includes seals and sea lions), carrying their young for as long as 15 to 16 months. (Seal and sea lion mothers are also dedicated, carrying their offspring for about 330 and 350 days respectively.) Walruses also have the lowest reproductive rate of any pinniped.
Whales and dolphins, also known as cetaceans, are known for their high intelligence, complex societies and peaceful personalities — it's no surprise that they also take a lot of care in developing their young. Although all species have different gestation periods, orcas have the longest period among dolphins (at about 17 months). Some sperm whales — the largest living predators — have been known to carry their young for up to 19 months.
Elephants have the longest gestation period of all mammals, carrying their young for nearly two years before giving birth. Long developmental periods are common among highly intelligent animals. Since elephants are the largest living and biggest-brained land animal in the world, there's a lot of developing for elephants to do in the womb.