Alonis was a planet, located somewhere in the galaxy's Alpha or Beta Quadrant. Alonis was the homeworld of an aquatic, vaguely humanoid civilization also called the Alonis.
Some Alonis served in Starfleet before their world's government became a Federation member state in the mid-24th century. (ST novel: Serpents Among the Ruins, DS9 - Mission Gamma novel: Twilight)
Alonis was attacked by the Borg during the 2381 invasion but was saved by the efforts of Benjamin Sisko, Elias Vaughn and the crews of the USS New York, the USS James T. Kirk and the USS Cutlass. (ST - Typhon Pact novel: Rough Beasts of Empire)