It’s Ambulocetus natans, folks. They could move around on land using contraptions called “legs” – as you may have noticed, modern whales typically don’t have these. Known from a single fossil found in Pakistan, they existed 48-47 million years ago and then literally disappeared into thin air (confirmed by witnesses at the scene)
I’ve heard there’s been a few studies suggesting it was fully aquatic, or at least couldn’t support its own weight on land. I’m not a whale worker though so don’t take my word on it
That may well be true if it was a transitional stage