The capital was under siege. Most of Paldea had already fallen. Yet the king was confident in his army and his trusty Mabosstiff, who patrolled the walls every night, barking whenever the enemy ventured too close; in time, the enemy would run out of supplies, and this, too, would pass. Until the night came when Mabosstiff also fell asleep. The barbarians overran the city and set it aflame, giving the city a brutal sack which ended Paldean Civilization for centuries. The Treasures of Ruin, who had slept through the battle and hide away to this day, bore a rage at these events which could not be quelled, so they cursed the entire Mabosstiff species to never smile again.
Although this story is by the best known thing about Mabosstiff, it took art historians and archaeologists surprisingly long to identify the canids painted on the walls of Paldea's capital. The colors and coat pattern were perfectly rendered within the constraints of ancient pigments, but a smiling, happy Mabosstiff simply does not look like a Mabosstiff. It is such an uncanny sight that it was first assumed to be a variant of Arcanine or Granbull or some now-extinct species, and the proof of its identity as Mabosstiff was a landmark paper in the field.
No Mabosstiff alive today was around for that tragic nap, but those pokemon old enough to have experienced Paldea's Golden Age have not forgotten this tale; anger over it is suspected to underlie a recent headline-grabbing incident with a Koraidon. Many Mabosstiff trainers have angrily denounced the injustice of punishing pokemon for the sins of their distant ancestors; a few have sought out the Treasures of Ruin themselves, in the hope of teaching their beloved pokemon to smile. They have been assured that the curse is not eternal.