Kaibutsu is the main antagonist in Ginga Densetsu Weed. He was originally a guard dog like Jerome and lived at the same facility, until one day he was chosen to be a test subject for a strange experiment. Using gene-splicing and intense doses of radiation radically altered everything about him, the tests 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 gave him the desire to survive and escape; eventually nothing in the laboratory could hold him, and he escaped after killing the scientists and several of Jerome's subordinates. He even attacked Jerome, and then fled into the mountains and Futago Pass.
After murdering numerous Ohu soldiers, he took over Gajou. More fighting and immense slaughter occurred, but eventually Jerome and his pack caught up with Kaibustu. After Weed had attempted to lure Kaibutsu out of Gajou, resulting in the death of Smith, he was attacked, killing the last of Jerome's comrades. Kaibutsu was eventually pushed over a cliff face by Weed, and was impaled upon a sharp branch above the river.
He has been mentioned briefly in the other episodes, particually episode 25 where a 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.
Kaibutsu's appearance has been radically altered from what it once was; measuring 12 feet long and weighing around 1,000 lbs. He has enhanced speed as seen when he was easily able to keep pace with a motorbike. The humans had also given him increased strength and vitality, which allows him to survive after being crushed by the rocks of Gajou.