The Fanged Beast lives in the Tundra, hence the white color of Lagombi's pelt. This camouflage works well in winter but when the ice and snow melts in summer it would be easily seen by predators like Barioth or Stygian Zinogre.
My theory is that Lagombi has a summer and winter pelage to get camouflage in the icy, white and brownish, earthy environment respectively. While the winter pelage is mostly white and dark brown at the belly, the fur in summer is brownish to black to blend in with the earth seen when the ice melted. Other known monsters that would have such color change aren't acquainted as most (like Barioth) don't need them or have already the right color (like Bullfango (Bulldrome)) and others don't rely on camouflage.