"P" - The Pangare, or Mealy gene
Pangare is responsible for 2, and possibly 3, color patterns:
That horses bearing the "wild donkey" coloring (light muzzle, light circles around the eyes, light flanks, light behind the elbows, etc.) exist is undeniable, but whether this is a single genetic modifier responsible has apparently not been proven to everyone's satisfaction.
Angelo, showing "Wild Donkey" coloring on Bay - owned by Jacqui Crews of Florida
Sponenburg clearly identifies a gene "P" for Pangare. The Pangare/Mealy gene "P" causes light areas around the muzzle, eyes, flanks, and insides of the legs.
Pangare on sorrel creates "Belgian" sorrel.