Octopus
Just as dolphins seem to appear as the standard marine "good guy",
the opposite roles had to be cast in the sea depths as well.
Tales of giant squids and octopuses harassing mariners seem to abound through all ages
- and they are also quite present in antiquity.
(Perhaps that's not unfair, given that in turn,
smaller versions of these mollusks have always been
a delicacy and thus hunting target for humans.)
Some conjecture that certain mythological sea monsters -
like the despicable Skylla - were derived from various such accounts. We might never know for sure,
but these animals do make their appearance on coinage, although in this case they might be
more a symbol of the reversed roles mentioned parenthetically above.