Mike

Mike (ミケ pronounced Mi-keh) is the guard dog of the Zoldycks. It is an enormous hound who has gone through strict training. Mike tries to memorize the stranger's appearance and scent. It shows no emotions and is hard to control. Other than the commands from Zoldyck family members, it listens to no one. The command that the Zoldycks gave Mike ten years ago is still loyally carried out, to kill all intruders. If something doesn't meet the requirement of the command, Mike will even attack its caretaker, without any hesitation.

Zoldyck Family arc
When Gon and company arrive at Kukuroo Mountain, they watch as two tourists force their way inside the property. Zebro the gatekeeper refuses, but the tourists take they key away from him, entering from the small gate. Moments later, a hand emerges from the small gate, taking out the skeletons of the tourists.

The three introduce themselves to Zebro and the gatekeeper acknowledges them as Killua's friends. But when Gon asks if Zebro can take them inside, he warns them about Mike. However, Gon is insistent, and Zebro takes them inside via Testing Gate. Zebro call out to Mike once all of them have entered. Inside, Mike is revealed to be an enormous guard dog. Zebro refers to Mike as a 'perfectly-trained guard dog'. The three are surprised and Gon admits that he is scared.

Trivia

 * Mike may not be the only guard dog of the Zoldyck family. In chapter 323 during a flashback, there are other two dogs which are in the same size of mike.
 * Mike is given the last name "Zoldyck" in the musical The Nightmare of Zoldyck.