As useless as this creature might be for your needs what about an Electric eel? It's an all natural abil. nothing magical about it?
Of course I could be out to lunch on this?
Here's what I could find;
Eel-shaped S. Amer. fish (Electrophorus electricus) capable of producing an electric shock strong enough to stun a human. The electric eel (not a true eel) is a sluggish inhabitant of slow freshwater, surfacing periodically to gulp air. Long, cylindrical, scaleless, and gray-brown, it sometimes reaches a length of 9 ft (2.75 m) and a weight of 49 lbs (22 kg). The tail region, bordered below by a long anal fin that the fish undulates to move about, contains the electric organs. The shock (up to 650 volts discharged at will) is used mainly to immobilize fish and other prey.
Or...
Electric Ray's
Any of the aquatic rays (families Torpedinidae, Narkidae, and Temeridae) that produce an electrical shock. They are found worldwide in warm and temperate seas, mostly in shallow water but some (genus Benthobatis) at depths greater than 3,000 ft (1,000 m). Slow-moving bottom-dwellers, they feed on fishes and invertebrates. They range in length from less than 1 ft (30 cm) to about 6 ft (1.8 m) and have a short, stout tail. They are soft and smooth-skinned, with a circular or nearly circular body disk formed by the head and pectoral fins. They are harmless unless touched or stepped on. The electric organs, composed of modified muscle tissue, are in the disk near the head. The shock from these organs, which may reach 220 volts and is strong enough to fell a human adult, is used for defense, sensory location, and capturing prey.
But this could all be a bit of dead issue. How does or would the cleric know of these animals. You may want to be carefull that you're not confusing player & char. knowledge