According to Wikipedia, it was still the Electric Eel.

"Elektron" is an ancient Greek word for amber, a substance that can hold static electricity, which of course became a word to describe that shocking sensation itself. So the idea of calling things that shock you "electric" predates exact knowledge of what electrical energy actually is.

However, it obviously only got that name when Europeans discovered it (in 1766). Apparently Native Venezuelans called it "Arimna", which means 'Something that deprives you of motion". This is according to The Atlantic so I have no idea if that's reliable.

"That slimy thing that hurts like fuck"