Once upon a time, there were no herbivorous mammals- but there were hoofed mammals - ungulates. Pakicetus is a member of an extinct line of ungulates that were carnivorous - so although they look like dogs, they're actually more closely related to sheep and hippos.

And yes, they are actually one of the basal ancestors of whales. How do we know? In the inner ear of Pakicetus is a specific feature endemic to their family group, which no other group of animals share- except modern whales! CRAZY.

These guys lived in what is now Pakistan 55-40 million years ago, and during that time, the Indian subcontinent had yet to plow into Asia (creating the Himalayas), so until then, these guys had a nice beachfront where they could frolic and eat fishies in the warm shallow lagoons. :D

Notice they already had the long fish-catching snout and high-rotated eye sockets on top of their head, perfect for breathing while swimming!