Fou

A cute animal that has taken a liking to Mash. It has traveled through each Singularity with Fujimaru and Mash. Normally, it freely roams Chaldea.

[Spoiler] He is an alternate iteration of Primate Murder — inhabiting those timelines wherein the Common Sense of Man pulses with life, and the Twenty-Seven Dead Apostle Ancestors do not exist as an organization. He is known as Beast IV.

Cath Palug is a member of a species of organism known as the Breed of Calamity (災害の種, Saigai no Shuu), which preside over the principle of "Comparison" (比較, Hikaku), the act of envying others' strength and regretting one's own weaknesses.

Originally a Beast of the Planet that became Merlin's familiar, Cath Palug was at some point subsequent to Merlin's sealing driven from the Isle of Avalon. In those worlds where the Common Sense of Man is insufficiently prominent, he came eventually to be known as Primate Murder — the pet of Altrouge Brunestud. [/Spoiler]