At some point before becoming a ghost, Cujo was a security dog at Axion Labs. It is unknown what happened in between the lab changing the security system and his first interaction with Danny.
Cujo first appeared at Danny's school and, after transforming, ran off to Axion Industries, with Danny in hot pursuit. Danny's attempts to restrain him only led to the lab being destroyed (resulting in Valerie's father losing his job).
Danny attempted to throw Cujo back into the Ghost Zone, only for the dog to escape again. He then began trying to tame Cujo, but the pair was attacked by Valerie, outfitted with her new ghost hunting equipment. Danny finally managed to locate the ghost dog's squeaky toy and Cujo happily gained liberation.
Life Lessons (Cameo)
Though he didn't appear in person. Cujo was pictured on Valerie's most wanted ghosts poster. His picture looked more like that of a regular dog.
Reign Storm (Cameo)
He later has a small appearance in "Reign Storm," as one of the many ghosts who escaped the Ghost Zone. Despite their previous bonding, he's seen briefly attacking Danny with the other ghosts. This could be a mistake on the writers' part that Cujo simply just didn't recognize him, mistook him for an enemy, or was merely angry that he never visited him. He was not seen with the other major ghosts later on, and it's unknown where he went.
Another interpretation could be that Cujo was furious at Danny being attacked and transformed in an attempt to come to his aid.
In that episode he was happy and cheerful as always and tried to lick Danny, Tucker, and Sam, but Danny turned intangible so only his friends were licked.
Cujo is a ghost dog with red eyes, a purple tongue, green fur, and black ears. He wears a spiked black collar with a silver Axion Labs tag. He can transform in size from a small puppy to a giant, monster-like dog.
Cujo is just a dog and does what dogs do: bark and play. However, since he's a ghost dog, his aggressive anger can be unlimited and very ferocious if he's provoked in any way. When angered enough he's been known to attack even his friends/allies.
Cujo is always happy but also very cautious when he wants to be. He had a strong sixth sense and can tell whether a ghost is strong or not.
Cujo viewed Danny as a 'master' or 'friend' since Danny is the one who found him and has thus been interacting with him. At first their meeting started off rocky as Cujo's continuous search for his missing toy caused several disruptions in Danny's life as well as the life of Valerie Grey, who would go on to begin hunting them and other ghosts as a result.
The two ghost-beings managed to bond during Danny's attempts to train him, only to find the dog has already been trained (during his life as a guard dog for Axion Labs). Cujo seemed to greatly enjoy the attention given to him by his new halfa friend and continued to return to the human world, presumably to seek out Danny. It is unknown whether Danny regularly visited his new ghost 'pet' or if Cujo followed his 'master' everywhere.
Danny mentioned in "Fanning the Flames" that he desired a puppy in his younger years but was forbade to own one by his parents. His joyful interactions with the ghost dog (primarily while in his smaller form) show that he does enjoy the animal's company. Sam admitted that the two are sweet together, though their ghostly attributes also make their interactions "freaky." Cujo was possible seen helpping Danny turn the earth intangable for the Disastroid to pass through but he possible was phasing.
Though generally an ally to Danny, his playful and aggressive nature often makes him almost as much of an obstacle or a threat as many of his enemies. He's even been known to attack Danny or his allies if enraged enough, though he usually means no harm.
Cujo unintentionally began Valerie Gray's hatred for ghosts and Danny Phantom when he destroyed her father's workplace while looking for his toy buried beneath the lab, costing her father his job. He continued to appear in several places where she is in the same vicinity as Danny, leading her to believe he was deliberately attacking her (he may have been tracking her as well due to her connection to Axion Labs).
Cujo may hold a personal grudge against Valerie as he does become violent when in her presence, tearing apart her lunch bag without provocation and growling at her menacingly, though there is not much proof or motive for this theory.
After these constant attacks continued to ruin her life, Valerie takes up ghost hunting weapons and attempted to eradicate both the dog and the ghost boy who she thought was connected to his attacks.
Powers and Abilities
- Intangibility, Invisibility, Flight: As a ghost dog, he has the same powers that all ghosts have. Cujo is the only ghost seen to be able to pass through the Fenton Portal even when it was closed.
- Transformation: Whenever Cujo is upset, he transforms himself (unconsciously) into a big, powerful, savage, and almost unstoppable dog with amazing strength that destroys everything in his path. Upon seeing Danny, he becomes relaxed and happy, returning to his normal and harmless form. When he is transformed, his normal abilities are increased and his tail also becomes green fire.
- Enhanced Strength: When Cujo is transformed, he becomes very strong and capable of bending metal, taking down titanium doors, and ripping out trees from their roots with incredible ease. Even when small, he can easily drag Danny around.
- Enhanced Stamina: When transformed, he is more durable than any normal human or dog. He has great Endurance and Stamina of musculature produces less fatigue toxics during physical activity than a regular animal.
- Enhanced Durability/Resistance: When transformed, his body is more resilient to damage than any human or living dog.
- Enhanced Speed: When transformed, he is faster than a normal dog. His super-humanly strong leg muscles also give him a certain degree of superhuman speed.
- Teleportation: He can teleport himself from one place to another at will, like he did when he appeared in the basketball game and when he disappeared after recovering his toy.
- Enhanced Senses/Sixth Sense: As any dog, his senses of smell and hearing are beyond normal human limits. He has 3 supernatural survival instincts (Smell, hearing, and vision) to sense dangers that happen moments before it actually happens. He also may have night vision like other dogs. He can tell whether a ghost is strong or not.
- Cujo also appeared in Nicktoons: Globs of Doom as the boss for Amity Park, being controlled and infected by the Morphoids-like blob.
- In the game Nicktoons: Globs of Doom, Danny called Cujo "Ghost Dog."
- Danny's choice of calling the ghost dog "Cujo" is a homage to the deadly dog from the movie Cujo, made famous by Stephen King.
- Cujo seems to be wearing an Axion Industries dog tag, a possible indication that he was one of Axion Industries' guard dogs and was drawn there to look for his chew toy in the area. This has lead to fan theories that he may have been put down.
- Unlike other ghosts in the series who typically oppose Danny, Cujo adores him. In all of his appearances however, his playfulness or ferocity made things difficult for Danny and his friends.
- Cujo is one of the very few ghosts who does not make a cameo in "Phantom Planet" turning Earth intangible.
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