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.
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.
In the timeline of Fate/Grand Order, he came to reside within the Chaldea Security Organization, journeying across the assorted eras of human history in the company of Ritsuka Fujimaru and Mash Kyrielight.
Cath Palug describes himself as the Beast of Calamity (災厄の獣, Saiyaku no Juu?) that bears the Characteristic (特徴, tokuchou?, lit. "distinct feature") of "ever exceeding the strength of an opponent" (相手より強くなる, aite yori tsuyokunaru?) -- growing unto maturation by the conflict that arises amongst Men, feeding upon their Envies (妬み, Netami?) and Regrets (悔しさ, Kuyashisa?).
Mash Kyrielight describes the Cath Palug as a squirrel-like creature -- "a cute animal," according to his official profile. However, he states that if he were let to normally mature, he would eventually degenerate into a "unsightly monstrosity."
Removed of the Civilization of Man, Cath Palug would be nothing more than a harmless animal.
He considers Merlin to be a "loathsome wretch," claiming that the man forced him from comfort of the Tower of Imprisonment (幽閉塔, Yuuhei-Tou?) in Avalon against his will. As such, he frequently attacks Merlin on sight (illustrated in Merlin's card for the April Fool's 2017 event, giving Merlin a flying kick to the face).
Garden of AvalonEdit
Upon witnessing Arthur's final moments, Merlin releases Cath Palug from the Garden of Avalon, insisting that he seek out "that which is beautiful."
Cath Palug is found by the Chaldea Security Organization and given the nickname of "Fou-kun" (フォウくん, ?). During his stay with the organization, he is permitted to freely wander their facilities.
Fuyuki: Contaminated City in FlamesEdit
E Pluribus Unum: Grand Battle of Legends in North AmericaEdit
Merlin uses Cath Palug as a conduit through which to temporarily reveal himself, so to lend aid to the protagonists in the battle against Cú Chulainn Alter.
Salomon: The Grand Time TempleEdit
In the aftermath of Mash Kyrielight's sacrifice, Cath Palug locates her soul in the place beyond life -- revealing his true name and his nature as a Beast. He then expends the majority of the Magical Energy that he's collected across the centuries of his life to transplant his Force of Providence into Mashu's soul, perfectly resurrecting her. However, the act deprives him of both his Sapience (知性, Chisei?) and Characteristic (特性, Tokusei?), leaving him as nothing more than a common animal. In the end, he congratulates Mashu and the people of Chaldea for defeating the Beast IV without violence.
Under certain circumstances, Cath Palug is capable of enacting a miracle that exceeds the scope of True Magic -- "the Complete Revocation of Death" (死者の完全な蘇生, Shisha no Kanzen-na Sosei?). More specifically, he was able to resuscitate Mash Kyrielight, but this was possible only due to the extra-temporal nature of the context of her death, the interior of Ars Paulina.
So to facilitate the process, Cath Palug consumed the majority of the Magical Energy he had accumulated across the centuries of his lifetime, using it to fuel the transference his Force of Providence to Mash, and overwriting the limitations of her remaining lifespan (3 days). However, as he wasn't yet an adult of his species, he was capable only of granting Mashu a lifespan comparable to that of a normal human -- at the cost of his own Sapience (知性, Chisei?) and his Characteristic (特性, Tokusei?) as a Beast of Calamity.
Taiki is the illustrator for Cath Palug.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
 Fate/Grand Order, Final Singularity, Chapter 15 - Beast IV
-- a relief. It's good that you haven't yet departed, Mashu.
Your annihilation was absolute.
You'd reached the termination of your life.
Even so, your consciousness yet lingers within this Void (虚無, Kyomu?).
Even so, you have labors yet to complete.
And therefore, I shan't tell you that "you've done a good job, and earned your rest."
The time, however, has come for me to bid you "bye-bye."
There isn't time to dally, and so I'll say it bluntly: I intend to bring you back to life.
I am that which presides over the principle of "Comparison" (比較, Hikaku?).
I am the Beast borne of the Characteristic (特徴, tokuchou?, lit. "distinct feature") of "ever exceeding the strength of an opponent" (相手より強くなる, aite yori tsuyokunaru?) which comes unto maturation by the conflict that arises amongst Men, supping upon their Envies (妬み, Netami?) and Regrets (悔しさ, Kuyashisa?).
The Beast of Calamity, Cath Palug (災厄の獣キャスパリーグ, Saiyaku no Juu Kyasu Pari-gu?).
In a different world, I am the one that would be called the Slayer of the Primates (霊長の殺人者, Reichou no Satsujinsha?).
Removed of the Civilization of Man, I would be a harmless animal.
And thus, I was once confined to a solitary isle, uninhabited by humans.
However, the Magus charged with the duty of caring for me was a loathsome wretch.
Forcing me from the comfort of the Tower of Imprisonment (幽閉塔, Yuuhei-Tou?), he released me unto the world without.
... but, it was thanks to that that I arrived upon Chaldea.
And there, I discovered you and Gudako.
-- y-you are --
Perhaps you find this troubling?
But, well, the Magus did once claim that goodwill is fundamentally something to be forced upon others.
Let that my actions be an emulation of that.
Expending the stores of Mana (魔力, Maryoku?, lit. "Magical Energy") that I've collected to myself across the centuries, I shall here enact a Miracle (奇蹟, Kiseki?) irreplicable even by way of the True Magics (魔法, ?).
"The Complete Revocation of Death" (死者の完全な蘇生, Shisha no Kanzen-na Sosei?).
Or rather, any deaths that come to pass within the Temple of Time (時間神殿, Jikan Shinden?) aren't in fact counted as applicable within reality.
I shall now be conducting a transference of my Force of Providence (運命力, Unmei-Ryoku?).
It shall persist to the extent that your remaining lifespan -- less than three days in duration -- will be entirely overwritten.
However, as I am yet to fully mature, I can grant you only a lifespan comparable to that of the Humans (人並, hitonami?).
And by the consummation of this act, I shall be as merely a beast.
Neither Sapience (知性, Chisei?) nor Characteristic (特性, Tokusei?) shall remain.
Until now, my responses to you in conversation have indeed held meaning, but henceforth, they may be little more than the cries of an animal.
But this isn't something you should unnecessarily concern yourself with.
From your perspective, after all, nothing will have changed.
But, I suppose I can't help but feel a bit of loneliness in that.
For this reason, I desired that my final act would be to bid you alone a personal farewell.
Goodbye, Mashu Kyrielight.
This journey that I've undertaken at your side was to me a refreshment.
It is owing to that that I was able to retain this form until the end, even as I might have normally degenerated unto an unsightly monstrosity.
W- wait --
Fou, -san -- Fou-san -- !
... Once, a certain Magus uttered these words on unleashing the Cath Palug upon the world.
"Go forth and seek beauty," he said.
I have indeed witnessed a beautiful thing.
There exist evils that may be defeated without the crossing of blades, and answers that be reached only absent of the spilling of blood.
I congratulate you, good folk of Chaldea.
By your efforts, the 4th Beast (第四の獣, Dai-Yon no Juu?) is here defeated.
- ↑ 2.0 2.1 Garden of Avalon - Chapter 06: In the Flower Garden, p.103
- ↑ 3.0 3.1 3.2 Fate/Grand Order - Fuyuki: Contaminated City in Flames - Prologue
- ↑ Fate/Grand Order website: Fou: A cute animal who the protagonist meets alongside Mashu. It seems to walk around Chaldea freely.
- ↑ Garden of Avalon - Chapter 00: At the Garden, p.013