Electricity was discovered around 600 bc, electric eels weren’t discovered til the 1740s (named in 1766). Natives in South America likely interacted with them, but we don’t have any records of what they called them. Most likely though we would’ve named them something to do with lightning if we hadn’t figured out electricity when we found them.
https://a-z-animals.com/blog/discover-what-electric-eels-called-before-electricity/