There are, it should be noted, a wide variety of “electric fish,” that have been encountered by many cultures, beyond the South American electric eel (such as electric catfish in Egypt and China, and electric rays in the Mediterranean). Those encounters with these fish produced numbing effects that were remarked upon and frequently used as medical therapies in both Ancient Rome and China, and electric eels were used in this way by natives in South America.
Hippocrates described the electrical Mediterranean torpedo with the name narkē, which is to say, the same origin as “narcosis,” which is a description of its numbing effect. Those Ancient Greco-Roman naturalists who tried to imagine what was causing the effect tended to think it was a form of poison.
The conclusive connection between the discharge of these fish and electricity was not made until the 1740s with the development of Leyden jars (basic capacitors for storing static electricity).
European understanding of electricity in part derived from studies of these fish and their organs in the 17th century. Alessandro Volta’s first battery (the voltaic pile) was described by its inventor as an “artificial” replication of the natural organs of animal electricity found in such fish, and there were deep debates in the 18th century about whether electricity was essentially an animal phenomenon or a physical one.
Anyway. From what we can tell, by the time Europeans described the South American electrical eel, they seem to have known about its electrical nature — the first reports of them in European scientific circles are from the 1740s. Linnaeus categorized it as Gymnotus Electricus in 1766. We have not found a reference to what it was categorized as before this, or what natives may have called it.
For more detail and information, there is a very nice article on the history of “animal electricity” here: Chau H. Wu, “Electric Fish and the Discovery of Animal Electricity: The mystery of the electric fish motivated research into electricity and was instrumental in the emergence of electrophysiology,” American Scientist 72, no. 6 (1984), 598-607.