Electric eel
The electric eel (Electrophorus electricus) was a species of fish from Earth that was capable of generating strong pulses of electricity with no harm to itself. 

Pavel Chekov compared the abilities of electric eels, giant dry-worms (of Antos IV), and another creature he was unable to complete the name of before Leonard McCoy interrupted him, to those of Apollo, in explaining his apparent ability to tap a flow of energy from an unknown source and channeling it through his body against the crew of the . ()

The  encountered a giant creature likened an electric eel while exploring the depths of the Monean Waters in 2375. The creature emitted massive biothermic discharges exceeding 500,000 volts, which disabled the Flyer. ()

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Earth animals