Mongrelamus is one of the original ten demons in the Krosmoz, and last to have been destroyed by Rushu in his power play. Although the Found Fragments in Dofus (MMO) say he was found and killed by Rushu, Krosmaster identifies him as the demon who possessed Goultard. Although he is always mentioned only in passing, he seems to be still alive in the Wakfu era, despite his defeat in the dawn of time.
According to Krosmoz.com, Mongrelamus (Médoroziam) was the last of Rushu's brothers to be killed by him when he decided to take over the entire demon race.
Rushu, lassé de partager la vedette avec ses frangins, finit par tuer 8 d’entre eux. Seul Médoroziam parvient à s’enfuir et trouver refuge sur une certaine planète abandonnée… Il s’y cache un moment, mais Rushu finit par le retrouver. Il le tue, avant de repartir, ni vu ni connu, pour la Shukrute.
The Krosmaster store clearly states that Mongrelamus is the entity possessing Goultard:
When Goultard is possessed by Mongrelamus the demon, he becomes a real killing machine!
Goultard le Barbare episode
Nothing is said of Mongrelamus to identify him in the special episode Goultard le Barbare, but shortly before the demon leaves his body, the possessed Katar enigmatically mentions that "he feeds on hatred"—thus explaining that his actions were only meant to enrage Goultard to allow (or ease) the transfer.
The DOFUS: Goultard Bazar manga makes a vague reference to the subject (translation ours):
While its origins remain obscure [...], one rumor persists: that it is one of the demons that were ousted by Rushu a long time ago. [...] One of [Rushu's rivals] came up with a ruse to prolong his fight for power: forcing himself inside a warrior's body.
— Goultard Bazar
Dofus Mag 1
The first issue of Dofus Mag, page 6, describes Mongrelamus as a "dog-headed god," who was forgotten and disappeared due to his lack of worhsippers (as gods in the Krosmoz draw power from worship). This mention of Mongrelamus as a god is strange since he is described as a demon where Goultard is concerned, and even one of Rushu's siblings.
Dofus Artbook Session 2
This Dofus artbook may have the most complete description of Mongrelamus to date, on page 44. It first describes the Universe of the gods and the universe of demons (see Krosmoz—"Universe" and "universe"), then introduces Mongrelamus as a dog-headed god who has since disappeared. It tells how he accidentally ripped the veil between the universes while burrowing a bone. To hide his blunder and his bone, he planted a platinum sign there that read "There's nothing here" in burning letters (this is referenced in Found Fragments I). Lastly, it reiterates that Mongrelamus disappeared because of a lack of worshippers.
In 2007, JeuxOnLine hosted a contest related to the open beta of the Wakfu MMO. One of the riddles, whose answer was "Mongrelamus," reveals or confirms some details about the demon. Our annotated translation of the relevant parts follows:
I serve the deceitful guardian of the demons' month [Djaul, protector of Descendre]
I am the one who buried his bone at the bottom of the Universe, under the rift to the demons' plane [see references in Dofus Artbook Session 2 and Dofus Mag 1]
No longer a god, serving my master Rushu [reincarnated as a regular demon?]
I wait to be freed, to finally be reborn free [possibly a reference to his being absorbed by Bilby when he was finally defeated by Goultard in the DOFUS Manga]