A Terrelian seapod was an intelligent lifeform native to the Terrelian homeworld. It was known for being very difficult to communicate with.
While Chakotay served in Starfleet, he once learned to communicate with a Terrelian seapod, a feat he figured prepared him to figure out how to talk with the nucleogenic lifeforms when they were attempting to destroy the ship, in early 2376. (VOY: "Equinox, Part II")