Sealsaycer
Its ancestor was a popular prey item for the Snoodcish and its descendants, and venturing onto land allowed it to reduce predation pressure. It hauls itself with its arms out of the water, skidding on its chest and slowly foraging along on the mud of the riverbanks. The Sealsaycer breathes by gulping air into its large, highly vascularized stomach, similar to the suckermouth catfishes of Earth. Similarly to a cow, its stomach is split into multiple parts with different textures. The "air stomach" is the first chamber, and is smaller than the "food stomach", with slight folds inside that make it somewhat gill-like. Due to the structure of its stomach, it is risky to feed gluttonously and continuously, as it may have trouble breathing on land.
Though Sealsaycers forage a few meters away from the riverbank, they must return periodically for cooler temperatures and moisture, particularly in the subtropical summers. It is also dependent on the water for breeding and molting its eye-scales.
The Sealsaycer has a more streamlined, seal-like shape. Certain parts of its skeletal elements are thicker and shorter, such as its "neck" chevrons, but most notably the "sternum" on its chest. The "sternum" is actually the "shoulder blade" for its chest arm, although the chest arm itself is nearly useless and so small it's often hidden by its other limbs.
As important as the sternum is for its movement outside water, it remains un-ossified. Instead, it gets the necessary stiffness and strength from thickened, almost scute-like saycerkeratin in the area. The shoulder bones, however, are fully ossified, and feature a more complicated shape of pits and crests, which serve as muscle attachment sites for its arms. It has one extra chevron in its tail-arm. It has thick, somewhat plastron-like saycerkeratin on its "chest" and underbelly, but it is not actually connected to its skeletal system.
Its sternum-arm and tail-fingers feature roughened patches of keratin (thinner than on the underbelly and chest). The three fingers of its arms are fused into fin-like structures. The arms are lobe-like, and have somewhat keratinous undersides, like "palms". This makes it somewhat less effective for swimming, but more suitable for hauling itself on land.
Its head is somewhat laterally flattened, seen from the front, something like a small lemon tilted on its side. Most have lost a pair of eyes on the lower part of its head: these eyes were too easily injured while hauling itself over sand and rocks on land, as it tends to lower its head when moving. However, the lost pair sporadically re-occur in the population. It has no eyelids. Instead, adults have a single "scale" of mostly-transparent skin over their eyes, superficially similar to the brille of a snake. This eye-scale helps filter out excessive light when it ventures onto land, and prevents the eye from drying out. It must be shed intermittently. It is still able to see without the scale, but not as well, due to being unable to deal with excessive light. It is poorly-adapted to nighttime vision, so it cannot simply leave the water at nighttime.
Like its ancestors, it has a radial fetal stage within the egg, but becomes bilateral before hatching. Its eggshell is made of living cells, arranged in a superficially placenta-like membrane, and unhatched babies feed on a yolk-like substance. Juvenile Sealsaycers resemble miniature adults, but for slightly smaller bones and less-developed musculature. They venture onto land only as subadults, when their bones and muscles are sufficiently developed.
Sealsaycers are largely herbivorous, but adults occasionally eat small, juvenile Twistworms and Wortopedes. They form only a tiny portion of the calories they eat, and function as protein supplements, especially when Amethyst Mosshroom numbers are low.