Gozu and Mezu

Gozu (Left) and Mezu (Right) are giant statues that guard the entrance to the afterlife. They faithfully protect this barrier to ensure that it is never opened and that no living being passes through it alive.

Inuyasha & Hakudōshi
When Hakudōshi begins looking for the pathway to the border between this world and the next, he learns of the Gate to the afterlife in the land of fire. However, he also learns that all those who appear before the guards of the gate turn into stone. He sends Kagura to Inuyasha in order to tell him about the gate, hoping to use him as a guinea-pig and see if he turns to stone.

The entrance to the Gate is littered with the bones of countless demons, as well as their body parts that have turned to stone. When Inuyasha and the others reach the entrance, a voice asks. "Do you wish to pass?" Inuyasha responds yes, and Gozu and Mezu spring to life. To Inuyasha's surprise, the Windscar has no effect on the guardians, as they explain that they cannot be cut by a blade of this world, and that only the dead can pass. Miroku uses his kazaana on Gozu and Mezu as well, and it also has no effect on the pair of living statues.

Inuyasha decides that even if he can't beat the guards, he can still force the gateway open. He cuts the chain that keeps the doorway locked and uses the Red Tessaiga to force open the doorway. Suddenly, an army of demons, led by Kagura, fly into the cave toward the now open gateway, hoping to get to the jewel shard before Inuyasha, when they become bathed in a strange light. All of the demons begin to turn to stone, and Kagura jumps off her feather just in time to avoid becoming a permanent fixture of the cave.

During the brief moments that the gateway is open, Kagome declares that she senses the sacred jewel shard within it, making it no longer questionable that the last shard is indeed in the border of the afterlife.

Sesshōmaru and the Tenseiga
Much later, Tekkei's head is severed, creating a river of blood, and Naraku, along with Inuyasha and friends travel to the border of the afterlife. Sesshōmaru smells the river of blood, and Kagura tells him the pathway has already opened and closed. She informs him of another pathway, one that even he might not survive, referring to the gateway in the realm of fire. When confronted by Gozu and Mezu, Sesshōmaru seems mildly amused by them, and when they declare they cannot be cut by a blade of this world, Sesshōmaru unsheathes his Tenseiga, and the statues kneel to him in respect. The Tenseiga has the power to recall and revive dead souls, and being guardians of the underworld, they would not oppose anyone who wields the power of life and death itself. Thus, Sesshōmaru is now the only one who can pass through this gate at will.

Trivia

 * Gozu and Mezu these characters are derived from the Buddhism and Taoism underworld guardians, (牛頭馬面). But unlike Gozu and Mezu have human appearances, Ox-Head has a a head of ox whereas Horse-Face has a head of horse. However other characters already appears with this forms early in the series.
 * Their names are also similar to Goz and Mez, the top two ogres of Hell/HFIL in the anime Dragonball Z, with Goz being the strongest and Mez being the fastest.
 * In the game Onimusha 2 there are two similar stone guardians in a gate in one of the levels.

Gozu_&_Mezu