The Triseraquins are ocean-dwelling creatures living on Earth.
The Triseraquins once lived on top of the seas until Aku sunk their city. Aku then told them if they wanted their city to be brought back up to the surface, they had to capture Jack. However, the Triseraquins realized that Aku tricked them and rescued Jack. They were seen in a cameo in Episode XLIII: The Aku Infection.
Fifty years later, they are among the known groups who witnessed Aku's triumphant broadcast. They retaliated immediately by storming The Pit of Hate with the Samurai's other allies and attacked Aku from the air using their Jelly-Whales. Some of them were defeated when Aku's forces (made from corrupted members of the Woolies and The Blind Archers' race) forced them off their airborne creatures using their enhanced jumping ability and claws. Some of them were killed by Aku's rain of spikes.
It is unknown how the past Aku's destruction affected their existence in the future. But it is possible that the only change would be that Jack had never met them. They tell Jack in the episode "Jack Under the Sea" that they were a social race living on the surface of the ocean, until Aku came and banished them to the bottom of it, suggesting that Triceraquins may have been on Earth since Jack's own time, if not longer.
Triseraquins are a highly-intelligent species, capable of building time machines, creating elaborate disguises and plots, and building cities underwater that can sustain non-aquatic life.
They are amphibious, meaning they are capable of surviving on land or in water, though they require water helmets to survive for prolonged amounts of time without moisture. They have a set of gills along their long necks and three, antenna-like protrusions on their heads. They are also incredibly tall (perhaps twice as tall as Jack) and have large hands with bulbous fingertips. They all have UK accents; Ringo speaks with an accent similar to Ringo Star, Guiness' accent is similar to Alec Guinness', and Connery's is similar to Sean Connery's.
They also have a unique pupil shape in form of an exclamation point. This is shown a few times during close-ups of their faces.