Let's come up with an amphibian race!

Back then, it wasn't a sea monster, but a sea serpent!

Exactly! A sea serpent is a leviathan. So everything will be fine.

Added 1 minute ago

No, a leviathan is a whale, so our Nessie is some kind of whale.

Leviathan is just an unknown monster. So it could be a sea serpent. It can definitely move on land.