A species of giant eel lives in the Seas of Pangaea and are referenced several times throughout the series. They are known to be vicious, but many monsters love the taste of their flesh.
The eels seen in Monster Rancher 3 are often even larger than your monster, so it is clear that they no longer grow that large since the Smoked Eel item that you can feed your monster in modern times is much smaller.
Some believe that the Electrieel mutated from this animal.