Neutrino

The Neutrinos were three kids from Dimension X: Dask, Kala, and Zak. They accidentally transported from Krang and Shredder's teleporter to Earth where they soon after became friends with the Turtles. Krang however mentioned that the Neutrinos aren't highly thought of in their homeworld.

After a brief stunt on Earth, the Turtles managed to become friends with the kids and helped them go back home. The Neutrino's returned a few times afterward, and a romantic relationship between Kala and Michelangelo was implied during those times.

The Neutrinos were apparently a race and the name wasn't just a group name for the three kids. In a later episode, Four Turtles and a Baby, it is revealed they are ruled by king Zenter and his wife and child. Krang tries to blackmail the small people, but eventually the Turtles stop him.

The Neutrinos are characters that have been invented for the 1987 animated series and haven't appeared outside of that show as of today. They appeared once in issue #3 of the Archie Comics, but that story was based on the adventure from the television show. Afterward, the Neutrinos were left out entirely.