Scientists think that whales are related to a lot of animals because there is no other animal that is bigger than them, besides dinosaurs. They had to have been made from a bunch of other animals combined. So what are those animals?? Here are a few major ones:

Indohyus (Meaning "India's pig" from the Greek words Indos, "from India" and hûs, "pig") is an extinct genus of digitigrad even-toed ungulates known from Econce fossils in Asia. This small chevrotain-like animal found in the Himalayas is one of the earliest-known non-cetean ancestors of whales. They are related because they are the earliest known one of the earliest-known non-cetacean ancestors of whales. They lived 50 to 48 million years ago.

Pakicetus (meaning 'whale from Pakistan') is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to the Indian Subcontinent during the Ypresian (early Eocene) period, about 50 million years ago. They are related to whales because skeletons discovered in Pakistan indicate that the animal had typical artiodactyl ankles and a typical cetacean skull. The later pakicetus started to spend more time in the water likely to find refuge from predators and to hunt fish easier. They lived around 50 millio years ago.

Ambulocetus (Latin ambulare "to walk" + cetus "whale") is a genus of early amphibious cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). They are related to whales because the Ambulocetus, which is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea. so that menas they were whales that could walk on land. They lived around 48-50 million years ago.

Basilosaurus (meaning "king lizard") is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago (mya). Basilosaurus shows the link or intermediate between whales and their terrestrial ungulate ancestors. The theory is that some land-living ungulates favoured munching on plants at the water's edge which had the added advantage of allowing them to easily hide from danger in shallow water. These crazy creatures lived 41.3-33.9 million years ago.

This changed because when they later Indohyus were born they mainly stayed in the water to gain refuge from their predators. So their bodies started to change into something that resembles fins.

This you now see in modern day whales.