He is a mutated dog who escaped from a laboratory and is vengeful towards humans. Though originally a dog of Japan mix breed, various experiments mutated him into a hybrid of dog, human, cow and various other animals' DNA. He continued to mutate even after his escape and grew in size and also grew a mane. He does not have a real name as he was only seen as an experiment and not a living being. The Ōu dogs simply called him "Kaibutsu" ("monster"), being unaware of his true origin. Jerome once called him "P4" to anger him, as P4 is the name of the laboratory apartment he was kept in.
Originally, Kaibutsu appeared to be an average-sized dog with black-and-brown fur, with markings similar to a German Shepherd. As he began to mutate, he grew in size and strength and his skin became very durable. He also began to grow a mane like a lion, which was still rather short at the time of his escape. However, it had grown much longer in 4 years. Kaibutsu's eyes are white or green with reddish scleroses, giving him a sickly and mad look (though, in the anime version, his eyes look healthier and clearer), and his body had grown much larger as well. His dark backside and the upper side of his tail is spotted. He also appears to have scales on his back legs, as well as chains which have been inserted into his flesh. Though Kaibutsu's skin is very durable, his weakness is the hair- and skinless area on the left side of his chest and stomach, which is quite vulnerable. The same area has a few small tubes stuck on his stomach, that's left from the experiments. Kaibutsu is around 12 feet (3.6 m) long and weighs around 1,000 lbs (450 kg). He has increased strength and vitality, while he also has enhanced speed, as seen when he was easily able to keep pace with a motorbike.
Kaibutsu was clearly insane and unstable after suffering almost all of his life. Due to his insanity, he is very aggressive and impulsive, but also has some cunning aspects. Though he seemed monstrous and evil, it was all result of the experiments done to him by humans. Because of it, he hated humans with a passion and thought they betrayed him. He attacks and kills harmless and defenseless humans, but do know that humans are dangerous, and thus stay away from that kind of humans. He believes humans and their dogs are cruel and that he is not. Indeed, Kaibutsu mentions that he does not want to make his opponents suffer longer than they have to. At first, he didn't want to harm dogs, believing to be one of them. However, Kaibutsu starts to see himself more and more as a monster instead of a dog. The only one Kaibutsu ever saw as his friend was Jerome and that is why he didn't want to kill him at first. However, Kaibutsu later explains that all the feelings of friendship, loyalty and compassion had disappeared from him along with his dog nature.
Ginga Densetsu Weed Manga
Kaibutsu was born to be an experiment and was soon taken away from his parents. He never got a real name and he was never allowed to leave Transgenic P4, the part of the laboratory he was kept in. His owner never seemed to view him as a living being and only saw him as an experiment. The first two years weren't very painful, but then humans started to use gene-splicing and intense doses of radiation, radically altering everything about him. Genes of cattle, human, and several other animals were put into him. The purpose of the experiments was to create organs for organ transplantation. The tests performed on him had caused him great pain and he found it difficult to adjust to what he had become. His only form of solace was Jerome, who would speak to him through a vent in the wall of his containment chamber. This went on for three years and it gave him the desire to survive and escape; eventually, nothing in the laboratory could hold him, and he escaped after killing two scientists and several of Jerome's subordinates. He even attacked Jerome and three days later he returned and killed the laboratory's owner.
He then went into hiding. When he escaped, he was still quite small and his mane was shorter, but his body kept changing even after the escape and four years later he was much bigger. Jerome and his 15 subordinates tracked him and tried to kill him, but without success. He escaped every time and managed to kill 10 of the dogs. Eventually, he arrived in Futago Pass, where he killed and ate all humans that came by. For a long time, the dogs and humans weren't aware of his existence and humans started to kill dogs, as the people believed the dogs were behind the attacks. Kaibutsu started to reside inside Gajō and was eventually found by the Ōu soldiers, who allied with Jerome and his subordinates. Weed and North try to trick him out of Gajō, but Kaibutsu kills North. Ken, and Kagetora and later Smith arrives and goes inside, where Smith fights the monster and tries to kill him by tricking him into causing a rock slide. Smith dies, but Kaibutsu survives and fights even more furiously than before. Jerome's last subordinates Robert, Rocca, and Heuler sacrifice their lives to wound Kaibutsu's chest, trying to reach his heart. Jerome tries to sacrifice himself and kill the Kaibutsu in the same manner, but Kaibutsu fights back and is now ready to kill his former friend, claiming that there is no dog left in him anymore. Weed, Ken, Kagetora, and Jerome himself finally overpower Kaibutsu and push him off a cliff. With the help of the water's current, Weed manages to move the beast and pierces him on a branch, which then kills him by piercing his heart. After his death, once he was found by the humans Daisuke and Hidetoshi, they realized he was the one who killed humans after finding some human hair inside him. However, Kaibutsu's origin was never found out by anyone.
Ginga Densetsu Weed Anime
Kaibutsu is the result of science’s research in gene manipulation. Kaibutsu’s genes and DNA have been manipulated to create the monster he is. He escaped from the scientific institute Transgenic P4 and promised to kill all the scientists who had been close to him. After escaping and hiding for 4 years, he arrived at the Ōu mountains. After murdering numerous Ōu soldiers, he took over Gajō. More fighting and immense slaughter occurred, but eventually, Jerome and his pack caught up with him. After Weed had attempted to lure Kaibutsu out of Gajō, resulting in the death of Smith, he was attacked, killing the last of Jerome's comrades. He was pushed over a cliff face by Weed. Kaibutsu fell with Weed, and Weed used the wind to impale him to a sharp branch above the river, causing his death. Kaibutsu has been mentioned briefly in other episodes, particularly in episode 25, where a supposedly dying Jerome wished to be reunited with all his former comrades and friends. Interestingly, he also expressed the desire to be reunited with Kaibutsu, suggesting that he had forgiven him.
- The kanji in Ginga Densetsu Weed Meishōbu Retsuden is 和犬の雑種 and stand for Dog of Japan mix. This is just what dogs in Japan were called before they were separated into breeds; the generic Japanese dog before classification (important note: although it's tempting to say it, 'original' dog is an incorrect term for this because it wasn't one breed that many stemmed from, it was a term for the group of dogs before they were separated into small [Shiba], medium [Shikoku], large [Akita] etc).
- Kaibutsu is not his name, it simply means monster in Japanese. And he is refered to as the monster.