The reason why marine mammals have horizontal tails is because they evolved from ancestors that had a walking phase.

Fish use lateral movements of their body to swim, necessitating a vertical tail that pushes against the water in these side-to-side motions.

Amphibians and many reptiles retained this lateral undulation even after transitioning to land. If you observe a salamander or a crocodile, you’ll notice their bodies twisting from side to side with each step.

Lizards exhibit similar movements, although they excel in territorial locomotion.

Now, let’s compare this to how mammals and dinosaurs walk. Instead of swinging their entire bodies from side to side, their spines flex up and down while their legs move directly forward and backward.

Marine mammals, having evolved from creatures specialized for walking, have spines that flex up and down but not side to side. Consequently, to transmit force efficiently from their spines to the water, their tails need to be horizontal rather than vertical.