The Merpeople are a race of near-extinct, intelligent, humanoid fish people. The Turtles first encountered their kind when, after a raft trip, they discovered a female of the mermen race (a mermaid?) dying of radiation sickness crawling to shore. They attempted to help her, but she died nonetheless. The matter was complicated when four mermen, thinking the Turtles to be her murderer, appeared and fought them. They eventually relented and took the body of the female merman back into the sea (TMNT Vol. 1, #28).
When exploring the sewers a considerable amount of time later, Leonardo found himself ambushed by another female merman, Rihana. Knocked unconscious and embedded into the wall, Leonardo was nearly fed to Rihana’s gigantic turtle companion, the Terrorpin. He managed to subdue Terrorpin, then discovering Terrorpin’s lair, and an egg within it. Both the Terrorpin and Rihana were presumably slain in the end, though Rihana managed to protect her egg (TMNT Vol. 2, #4 & 5).