The Deirr were a Federation member species from Deneb Kaitos IV. They resembled upright slugs made of wet brown leather, but they were eight feet long and five feet thick. They had one multipupiled eyeslit, below which was a vertically-oriented mouth containing many teeth. Beneath the mouth was an array of tentacles of varying sizes used to manipulate objects. Their laughter sounded like bubbling gastric juices. (TOS - Rihannsu novel: My Enemy, My Ally)

A Deirr was amongst the Federation delegation to the planet Kropasar in August 2375. (SCE eBook: The Future Begins)