science

All of them have decent wiki pages. These are basically in chronological order and while not necessarily a direct line of descent show roughly how cetaceans went about returning to the water. The first few were mostly found as fossils in India and Pakistan.

1. Large racoon-size. Thought to be very comfortable in water but didn't necessarily live there most of the time.
2. Wolf sized. This one probably actually hunted in the water.
3. Seal sized-ish. Likely lived mostly in the water like a hippo or mammalian alligator. Eyes and nostrils have moved up to allow breathing and looking around above the waterline while staying mostly submerged.
4. Similar, but even more fully adapted to life in the water, and while it likely came ashore to give birth, it would have been awkward an walker.
5. Even further specialized, and up to 400-600kg. Artistic representations look like a nightmare fuel combination of alligator and manatee.
6. Fully aquatic now, and the size of a very small orca. Widely distributed fossils in what were shallow seas.
7. A small "macroraptorial" sperm whale, basically filling the niche currently occupied by orcas, hunting large marine vertebrates.