Double Rear Dew Claw it’s a “Trademark” of Great Pyrenees dogs.
Pyrenees were bred to roam vast terrains with steep slopes and guard the flocks.
Double dew claw is used as a "thumb" when climbing the rocks and mountains,
giving dog’s better stability.
Double dew claws are mandated in the breed standard.
In a Show Ring Pyr is disqualified if there is
“Absence of double dewclaws on each rear leg”
Although double dew claws can show up in any breed,
They are most common in this guardian dogs:
Anatolian Shepherd Dog
Estrela Mountain Dog