The Ai Chun

Poul Anderson, World Without Stars, XI.

The downdevils or "Ai Chun" are:

amphibious;
"...pinnipeds of a sort..." (p. 72);
twice human length;
several times as bulky;
sleek headed;
possessed of luminous chalcedony eyes and fingered flippers;
able to sit up on land;
rare intelligent sea-dwellers, like Terrestrial dolphins or one of the two Starkadian races in the Technic History.

Argens' Evolutionary Speculation
The old planet of a dim sun slowly lost some of its hydrosphere.
The growing dry land was colonized not by modified fish but by organisms already breathing air (how come?), "...with high metabolism and well-developed nervous system." (p. 73);
new conditions and thermal quantum processes stimulated evolution;
the planet lost its satellite;
the Ai Chun's eyes are inadequate for the current long nights;
they hate the galaxy which brightens the sky on alternate nights.