The Melthusians were a spacefaring species.

A Melthusian captain, Bela Tyken was the first to encounter a spatial anomaly, which was later named Tyken's Rift in his honor. (TNG: "Night Terrors")

This species was only mentioned in dialogue. However, they were one species (along with the Cardassians, the Bajorans and others) which Herman Zimmerman referenced as having participated in the construction of space station Deep Space 9, in the way the station was originally conceived by the creators of Star Trek: Deep Space Nine. (Trek: Deepspace Nine, p. 52)
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