Kaibutsu was a dog chosen for experimentation by a group of scientists. His body was modified and with great pain, Kaibutsu suffered every day for five years from experimentation and tests. Jerome, one of the security dogs from the lab would talk him through the pain every day through a vent. No other dog gave any notice to Kaibutsu. Kaibutsu. Over time, Kaibutsu's body grew to an abnormally large size and his hatred for humans grew as well. One day, he managed to escape from the lab. Jerome was one of the security dogs sent to find and hunt down Kaibutsu after his escape. In the wild Kaibutsu would hunt and eat humans and the people of Japan started hunting the wild dogs including the dogs of Ou because most of the people who saw Kaibutsu didn't live to tell the story.

   While living in the wild, Kaibutsu killed Tokimune (Gin's substitute) and everyone thought it was Gin, including Weed. Smith explained to the Ou dogs who Tokimune was. Later on Noss (One of Jerome's subordinates) is killed by Kaibutsu when trying to lure Kaibutsu out of Gajou for an ambush attack. Smith sacrifices himself while fighting Kaibutsu, he latched onto Kaibutsu as Gajou's heavy rocks fell on top of both of them. Kaibutsu survives and next day tells the dogs of Ou his back story while talking to Jerome. He can't see why he should be loyal like Jerome to humans, when humans tortured him. Kaibutsu owes Jerome so much for talking him through the pain all those years and doesn't want to kill him. He snaps when Jerome calls him P4 and Kaibutsu kills Heuler, Robert and Rocca. All of the dogs attacked the weak point in his chest, tearing it open wider with each attack. Jerome grabs ahold of Kaibutsu's heart with an attack but is uffocated by Kaibutsu massive jaws.

   Ken and Kagetora hold down Kaibutsu and Weed pushes Kaibutsu with his comrades off of a cliff. Using the wind, Weed moves Kaibustu around so he lands on a log below with a sharp branch sticking up. This kills the massive, experimental dog.

"Kaibutsu" means "monster". He never get a real name as he was taken by scientists only to be a test subject.

Monster was born to be an experiment and was soon taken away from his parents. He was never allowed to leave Transgenic P4, the part of the laboratory he was kept in. Genes of cow, human and several other animals were put into him. The purpose of the experiments was to create organs for organ transplantation.