Phillip’s winter coat.
 
Descended on his dad’s side from the pegasi of Fólkvangr to the far north of Hyperborea’s shell, Phillip grows a shaggy black coat that covers most of his body when it starts to get cold, which annoys him to no end. The long coat, combined with his tendency to stack on quite a bit of weight at the onset of winter, makes flight difficult until it sheds in the spring. While the identical colours make it seem that his mane is covering his body, his mane itself doesn’t grow all that much aside from becoming longer.
 
While most Fólkvangrian pegasi undergo a similar process, most of them don’t substantially change colour like Phillip does.
 
Summer coat here.