In Japanese Buddhism, Gozu and Mezu are the demon generals who guard the gates of hell. They appear as terrible oni with animal heads; an ox head for Gozu, and a horse head for Mezu. They are extremely powerful and have the strength to move mountains. They are servants of Great King Enma, the ruler of hell, and are among the chief torturers and punisher's of the wicked.
Gozu and Mezu are the first demons that one encounters upon entering hell. Should a person manage to escape from hell, Gozu and Mezu are sent out to bring them back.
- Gozu on Yokai.com