This could have been whales today and they could stomp onto land like a hipporilla/wangaroo but they had to migrate further into the ocean and their hind legs become weak and useless by the fourth-gen whales.

If they even used their hind legs to flap and swim they would have some gargantuas propelling speed for the large mass of some whale species and essentially be a living under-water missile the size of the Empire State Building.