Why is there a difference in the rotation of the tail fin in fish compared to marine mammals?

Question

I've been thinking about this one, but I can't seem to find what causes this difference.

All fishes that I've seen have their tail fin positioned vertical:

But all the marine mammals I know have their tail positioned horizontal:

Why is there this difference? Is it because it's mammals compared to fish? Or could fish as well have horizontal tail fins?

Answer 1

Cetaceans (i.e., marine mammals) evolved from certain ancient land based mammals, thus the tail is essentially convergent evolution of the tail function.