From Guild of Archivists
Sunners swimming underwater.
Found in Riven

Sunners are, by the time we see them in Riven, an extremely endangered species. They were hunted by the Wahrks as well as the Rivenese when food began to grow scarce in the lake on Jungle Island. Running out of alternatives, the natives turned to hunting the Sunners, which were relatively harmless creatures. As a result, Sunners are extremely skittish around other beings.

Sunners have often been described as something of a cross between a baleen whale, a platypus, and a penguin. Their large beaks have baleen in them, which they use to filter plankton out of the sea water for food. They are called Sunners because, in the game, that's exactly what we see them doing: sunning themselves. It is unknown what the Rivenese called them.