Everything there is slittery, minotaur lamias, people lamias and even flying lamias and swimming lamias.
Why did so many animals lose their back legs in this island?
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The symptoms of phocomelia syndrome are undeveloped limbs and absent pelvic bones; however, various abnormalities can occur to the limbs and bones. Usually the upper limbs are not fully formed and sections of the "hands and arms may be missing." Short arm bones, fused fingers, and missing thumbs will often occur. Sometimes hands or fingers will be present but limp due to having no bones or being loosely attached. Legs and feet are also affected similarly to the arms and hands. Individuals with phocomelia will often lack thigh bones, and the hands or feet may be abnormally small or appear as stumps due to their close "attachment to the body... When an individual is born with phocomelia due to drugs or pharmaceuticals, it is known as thalidomide syndrome
Thalidomide is the classic teratogen that causes loss of limbs; the web has plenty of images of afflicted children, many of whom were developmentally normal except for the missing limbs.
There is a thalidomide-like teratogen on your island - perhaps a plant or fungus compound, or something in the water. This is how such a wide spectrum of creatures is affected - these are all vertebrates and share a body plan, and so are similarly affected by a teratogen that affects limb growth. They are recognizably kin to their 4-limbed conspecifics on the mainland because not that much evolutionary time has gone by - genetically they are the same species.
These creatures might also have issues with forelimbs, according to what you need for the story.
As re the kitten: it is not clear to me from the story if this was from a teratogen or a mutation. My bet is a teratogen. The image seems less heartrending to me than the kids missing arms and legs from thalidomide. Apparently these kittens are doing fine.