It is unknown when the Aurons learned to domesticate the giant eels, but it was most likely after the dispute between their tribe and their banishment from the island of Auron and before Doran the Dragonlover made his way to the Pirran sea. Since most of the Aurons' boats were used in construction of the great raft, the eels became a necessity for Auron guards and civilians to traverse the underground seas.
The Isle of Illusion
When Lief, Barda, and Jasmine arrived in the Auron sea, they were stopped by a battalion of Auron guards riding on giant eels before they were escorted to the great raft.
Later, when Penn brought the companions to the island of Auron, they drifted too close and triggered the web of the Arach. When two Arach arrived to destroy their boat, Penn called forth two giant eels to bring the companions back to the Aurons' raft.
Penn rode an eel when she saw the companions off on their journey to the island of Keras. Later, Aurons rode giant eels to greet the companions, Emlis, and the Deltoran slaves, who had been brought back to the caverns by the united powers of the Pirran tribes.
The giant eels appear similar to other eels; they have long serpentine bodies covered in silver scales, large eyes, and long jaws filled with needle-like fangs. How large the eels grow is unknown, but on average they can allow for three people to ride behind their head.
The giant eels are very loyal and helpful creatures. They willingly allow Aurons to ride on their backs and don't lash out against outsiders unless told to. The eels are also quite cooperative, as they allow Auron guards to carry flaming torches on their backs without submerging to douse the flames. They are also able to be called and commanded by Aurons.
However, the eels are not completely subjugated to the Aurons, as they would not swim close to the Isle of Auron while there were Arach about.