A very recent phenomenon caused by the instability of the planar boundary affecting the denizens of the Material Realm.
Transmission & Vectors
Mutations appear to affect living creatures, including plants, randomly with random effects. It is currently unknown why some creatures are affected while others are not. The only pattern is that celestial creatures don't seem to be affected; only creatures that originate from the Material Realm may experience mutations. It is not contagious, though.
Most likely, these mutations are caused by the breakdown of the planar boundaries bringing the celestial and spirit realms into violent and sporadic contact with the Material Realm.
Changes usually start small, such as a discolored patch of skin. Then, the change spreads and grows, becoming a part of the creature's body as if it had always been there. The process is not painful when the change is gradual, but occasionally, the changes occur so rapidly that it tears muscle and breaks bones in the process, causing extensive internal bleeding. Creatures who undergo the rapid version of this process rarely survive it.
The only consistently effective treatment is to cast "protection from evil and good" on the afflicted at as high a level as possible.
One of the earliest instances of planar mutation that this author is aware of is the appearance of what was later called "tree disease" in Rockport and Phandalin on the coast of the Western Reach. At first, it was thought that the patches of bark that appeared on people's skin was caused by a new spore or virus. However, with further study, it was found that the condition did not affect everyone in the area equally nor was it transmitted to those in close contact with the afflicted. In addition, after the initial outbreak, the phenomenon stopped as abruptly as it had started. This led to further study of mutations in other creatures and scholars found that a wide variety of physical changes were happening to plants and animals in the surrounding area. The main commonality was that, in each case, the creature took on recognizable traits of another creature. For instance, scholars found an entire warren of rabbits with sharp teeth and claws, orange fur with black stripes, and cat-like eyes.