How Aliens Would Reconstruct The Animal - spooky scary hippos

Agreed, Hippos are one of the most territorial, mean, and dangerous animals out there. They kill about 500 people a year. You do NOT fuck with a hippo.

Side note, they don't swim, they're so heavy that they instead stop floating and then sink to the bottom of the river and RUN.

It's not that hippos are too heavy, it's that they are too dense to swim. Elephants are heavier but less dense, hence they can swim (albeit slowly). This doesn't make them any less dangerous, however.