The Yamainu (山狗) or Mountain Dogs (Yen Press translation: "Wild Dogs") are a counter-intelligence force under the control of "Tōkyō". Okonogi is the leader of the squad, although Takano can act as a commanding officer. Though they are initially there to protect Furude Rika (under codename 'R') and those working at the Irie Clinic, they are bribed by Takano Miyo to help her carry out her plan to destroy Hinamizawa. However, when her schemes fail, they reveal that Nomura from another faction in "Tōkyō" had paid them more, and that they were never under Takano's orders.
Known members and units
They appear in Onikakushi-hen, when a paranoid Keiichi runs away from Ryūgū Rena, they knock him out when they came across him. They were forced to retreat when Rena came looking for him. Rena then took the unconcious Keiichi back to his house, and called both Mion and Irie Kyōsuke to help him out. Irie then ordered the Yamainu to keep a close watch on Keiichi when Rena told Irie about Keiichi's sudden suspicious and erratic behavior.
However, the Yamainu's existence only drove Keiichi to be completely sure that everyone in Hinamizawa, regardless if he knows them or not, is out to kill him, due to the fact that there was a "needle" in one of the ohagi that Mion and Rena made for him, and that on that same morning, the Yamainu came close to running Keiichi over with their van, though the events were probably very exagerated by Keiichi's paranoia, as the van honked before passing by him.
They attempt to secure Keiichi after he murders Mion and Rena in his home, however Keiichi escapes and ends up dying in a phone booth.
Okonogi and one of the Yamainu's soldiers kidnapped the grandson of Minister M. Inugai. They hide in a house of Takatsudo and summoned Irie to examine the child, in case he was sick. Akasaka and Ōishi, investigating the kidnapping, encounter and fight the Yamainu. Ōishi was knocked out, and Akasaka was shot by Okonogi in the shoulder. Nevertheless, they eventually manage to "rescue" Toshiki.
Along with being involved in committing the GHD, the Yamainu are also required to silence any survivors of the disaster and/or anyone from Hinamizawa who was outside the area of the massacre. In the sound novel, manga TIPS, and the anime's Yakusamashi-hen, the survivors either died due to suicide or "symptoms similar to acute heart failure. It is revealed in Yakusamashi-hen that a Yamainu soldier may have suffocated the victims to carry out a silent murder and lead doctors to diagnose the death as from acute heart failure. Hōjō Satoko in Yakusamashi-hen, Sonozaki Mion in Taraimawashi-hen, and Maebara Keiichi in Tatarigoroshi-hen were all observed by the medical personnel, some of whom may be connected to "Tōkyō," to have direct conversations with either a police detective or a reporter, and then died of "acute heart failure" shortly after they leave.
The Yamainu play a major role during the events of Matsuribayashi-hen when, early on the day of Watanagashi, Furude Rika is reported to be dead. As this throws Takano's plans of starting the GHD astray, she realizes that the report is faked. The Yamainu's spies in the prefectural police department attempt to confirm or deny this; however, they are stopped by Ōishi Kuraudo . Despite the report never being found out to be fake, Takano initiates heavy surveillance of Rika's house with about four Yamainu observers. During this time, the Yamainu capture Tomitake Jirō from a hotel in Shishibone City and bring him back to the Irie Clinic while Takano interrogates Irie.
While questioning Irie, Takano becomes increasingly frustrated with a logical impossibility presented to her: before Watanagashi, Rika's house was under surveillance by the Yamainu, who confirmed no one had entered or left the residence besides Irie. Irie claimed that Rika was in the house with a cold; yet, her body turned up at the Okinomiya police station. After a call from Nomura, Takano orders the Yamainu to place Irie under heavy surveillance.
Irie notices that Tomitake was captured and escapes his clinic by car. A group of Yamainu pursue him in one of their vans along the forest road. One of them successfully shoots Irie's tire. When they attempt to secure Irie, they are stopped by Tatsuyoshi Kasai with Sonozaki Shion, but are able to track them based on a tracker hidden on Irie. They pursue them to the Sonozaki manor.
Once Irie, Kasai and Shion arrive at the Sonozaki manor, several other backup vans along with Okonogi arrive to secure the estate. Here they make use of plastic explosives to break down a large door to the Sonozaki torture chamber as well as night vision equipment to see in the dark due to having cut the electricity (along with the phone lines in Hinamizawa to prevent contact to the police/the Banken) . While invading the chamber they used hand grenades and possibly assault rifles to secure the area. After abducting Rika, some of the men go back into the basement in order to eliminate the rest of the group protecting her, as the few along with Okonogi go to bring Rika back unconscious. A few men are injured as Akasaka Mamoru rescues Rika and nearly incapacitates Okonogi. The forces retreat while the opposing side sets up a plan to use Irie's tracker bug as a decoy while sending the club members into the mountains with it. The Yamainu are unaware of this plan and subsequently pursue the club with most of their squad, leaving about eight guards in the Irie Institution.
The control center van containing Major Takano and the main radio hub as well as around four other vans head out into the mountains, facing off against the club with taser guns. These prove to be useless against the club's ability to work together and Hōjō Satoko's prowess with traps. Meanwhile, Irie, Kasai, Akasaka and Shion infiltrate the Irie Institution, incapacitate the eight guards, ruin the Control Room's main interface and place Hōjō Satoshi in a safer area for recovery. After Tomitake is released, he and Akasaka escape to Okinomiya after facing off against two Yamainu securing the borders of Hinamizawa armed with RPGs and assault rifles. Tomitake calls in the Banken, another military organization under the control of "Tōkyō" for assistance in keeping the research secret.
Eventually, most of the Yamainu in the mountains are wiped out. Okonogi shows his skills in melee fighting against Mion but is defeated. He considers the entire ordeal to be over, and he reveals to Takano that due to this failed scenario Nomura is going to have her be killed. He tells the rest of the Yamainu to surrender to the incoming Banken while offering a handgun to Takano for the purpose of killing herself. Afterwards, Okonogi surrenders along with the rest of his troops while assuming the entire incident will be wrapped up neatly by "Tōkyō."
They also make an appearance in the first episode of Higurashi no Naku Koro ni Rei where they were asked by Takano to remove Keiichi's trunks. However, Ōishi calls his police squad. The Yamainu realize they don't have their weapons with them due to the fact that they are hiding in a swimming pool.
The male members of the Yamainu look very similar. They all wear the same gray uniform, a gray cap and black shoes. In addition, most are always armed. If they are traveling, they usually go around with a gray-white van or truck. At the windows of the cars are usually very dark, so the victim can not identify. A female member appeared as a nurse in Yakusamashi-hen. In some of the manga arcs, they appear to dress in casual clothes.
- All of the Yamainu Codenames are the names of birds.
- 大鳥 (ōtori) means fenghuang, a giant bird in Chinese mythology.
- 雲雀 (hibari) means skylark
- 鶯 (uguisu) means Japanese nightingale
- 白鷺 (shirasagi) means white heron
- A Dog's Life: The name is based on the believed extinct Japanese wolf. This is a Japan Times article on how they may be the ancestors of domestic dogs, while this suggests that, perhaps, they may not be extinct.