They were a sentient, air-breathing cetacean species. A long-lost race known only as the Shapers used genetic modification to create the Psyll, likely for use as sea-farming labor. They numbered over 4.4 billion in the late 23rd century.
Orion colonists came to Volgas later, and shared in the benefits of Psyll talents. Orions and Psyll had a happy and peaceful relationship and worked closely together as partners. This closeness compromised local Orion culture and separated Volgasan Orions from those off the world.
Wrecked spaceships were rumored to lie at the bottom of Volgas's ocean, and offworlders dived to hunt for them and discover their secrets, but both Psyll and Orions were both apparently entirely uninterested in them. They did not stop anyone from exploring for them, but nor did they offer any aid. Some said it was the Psyll who particularly discouraged exploration, but couldn't say why they, an air-breathing race who couldn't go so deep on their own, should care. (FASA RPG module: The Orions: Book of Common Knowledge)