This is just because English sucks, or English speaking people suck at naming things. Let me show you how it's done:
In Dutch:
Horseshoe crabs are called "dagger crabs", and look what it's dragging behind.
Cuttlefish are called "ink fish", and tadaa.
Jellyfish are "kwallen", which means roughly "annoying person", and they're pretty annoying.
Bald eagles are "American Eagles", you're welcome.
A sand dollar is called a "sea coin", because of where it lives and what it resembles, which is way more accurate.
And a fly is still a fly.