The Lothra were a hydrogen-breathing species.
In 2370, while discussing interspecies relationships with Ensign Melora Pazlar, Jadzia Dax used the story of a Lothra and an Oxygene who had fallen in love and were only able to spend forty minutes a day together without their breathing apparatus to illustrate how Pazlar and Doctor Bashir could find a way to make a relationship work. According to Dax, the couple were together 57 years despite the difficulties. (DS9: "Melora")
This species was only mentioned in dialogue.
StarTrek.com speculated that this species hailed from a star system with the name Lothra and/or a hydrogen-atmosphere homeworld named Lothra. [1]