Cthulhund is the most popular fandom nickname of the dog beast appearing in chapter seven, which never received a canon name. It is also called Rashdog, Spider-Dog, Wormball, etc. etc..
Cthulhund was shown in three different "forms" during the chapter: As a dog-based beast with relatively few deformations most of the time; in a modification where extra rib-limbs had roughly half of the normal form's hide stripped clean off the body; and finally, there was a moderately scary dog spirit to be seen in the dreamworld that is believed to be the soul / dreamworld avatar of Cthulhund.
The "normal" form was that of a rather large but slender, if neglected, dog reminding many readers of a Borzoi. Its fur was matted to the point of its jaw possibly having been fused together, the hide appeared split open along most of the length of its spine, blood seemed to be dripping out in places, most notably the tail tip, and - true to Sigruns statement "you can always tell [wether an animal has been turned into a beast or not] from the eyes" - what seemed to be bunches of worms or tendrils were sitting next to or even piercing through its eyeballs. Minna confirmed that Cthulhund had been beastified in year 0, and at least before turning to attack Emil, it wore a collar with hearts embossed on it.After the fight started and some of Emils bullets had hit, the beast hiding inside Cthulhund "activated" a series of ribs to become (much longer) legs, rip the hide off the head, forelegs, and upper chest, and give chase faster than the original legs - the front right one having been pierced by a bullet - could have.
Finally, Reynir met a dog in the mages' dreamspace that is commonly believed to be Cthulhunds avatar there. It has an outright skull instead of a head, a single heart shape appearing on its throat, pointy protrusions along its spine, and seems to be noticeably chubbier than Cthulhunds real world body, but didn't look particularly monsterific apart from its skull-head.
Personality and Behavior Edit
Cthulhund was seen tracking mommy-cat and her kittens, and having mommy-cat "treed" up a ferroconcrete pillar in The Fictional School, but it was never shown that it actually acted aggressively towards them. Likewise, Cthulhund didn't seem aggressive towards any of the humans most of the time, rather looking like it was seeking their help. The available information on dog beasts supports that some of them may show mind and behavior of their former dog selves. (Cthulhund doesn't seem to fit into one of the noted subtypes, though.)
While Cthulhund did jump towards Emil before a shot was fired, it is unclear whether that was already an attack. Bullets hit before Cthulhund landed again, however, and it switched to its "attack form" and gave chase after Emil into the school's corridors.
The more interesting mood swing happened when Cthulhund had actually closed in to the point of opening its jaws to bite - and swung its head around so as to snap at the empty air, instead of harming Emil. There was extensive discussion of what might have triggered the switchback - see the three most important theories below.
Once the attack was aborted, Cthulhunds rib-legs put the hide back on, hid under them, and Cthulhund ran out of the school, only to reappear near the cat-tank later and let Emil kill it willingly.
Good Doggy Theory Edit
Since it is confirmed that dog beasts may show signs of their former dog character, there is the possibility that Cthulhunds old behavior resurfaced and prevented the rashed monster it has become from hurting one of the humans it knew from way-back-when. It would imply that the life of such a "conscious" dog beast may be a decades-long struggle against its illness, but it is also unlikely that positive proof for it will become available.
Enlightenment Theory Edit
Moments before Cthulhund aborts its attack, a patch of sunlight shining through a hole in the ceiling can be seen on its head and its eye loses the cloudiness. This has led readers to believe that direct (sun)light plays a role in suppressing the beast mindset and allowing the original mind to resurface.
However, Cthulhund was seen trailing mommy-cat in the sun before the rain started, and switched to start the attack inside the hall, which was rather more sunlit than the corridors where the hunt and the switchback took place. Also, sunlight is known to post-Rash humans as a disinfectant; it seems unlikely that a monster-mind-suppressing effect would have been overlooked for 90 years.
Family Ties Theory Edit
Last not least, it has been suggested that Cthulhund may have come from a kennel that supplied the post-Rash Västerström family with its (now lost) wealth, possibly starting with the prologue Västerströms' dog Bosse, and that Cthulhund aborted its attack when it came close enough to recognize Emil as a family member by scent.
Minnas confirmation that Cthulhund became a beast in year 0 (and, supposedly, on Amager) has effectively disproven this theory.
- While we never saw Cthulhund attacking any of the cats, its intervention caused the death of all except Derpkitty.
- Emil killed Cthulhund, apparently instantly, by plunging a dagger into its brain, further supporting the rule of thumb that destroying the heads/brains is the gold standard method of killing grosslings.
- Cthulhunds begging "nyuuuuu" whine has become a meme of its own.