The Skull Guard is a character in Moshi Monsters, having made merely a one time appearance in the very first Super Moshi Mission.
Upon meeting them they complain about a key rattling in their head and dare you into reaching in to retrieve it, as they assume you to be too scared to do so. Their head is filled with Creepy Crawlies that you force out of the way in a minigame, and manage to get the key out, through Skull Guard's eyesocket. Relieved that they can think more clearly without the key and perhaps the Crawlies more organized, they warn you that "Master" will be back soon and that you better hope he does not catch you poking around.
In Big Chief Bill's hut in Go Do The Hoodoo, their flash files are used as lamp post decoration. That said, the entirety of Hoodoo Central is full of skull lanterns, all of different design. If all of them are also sentient and capable of speech is unknown.
They appear to be unaware of their purpose, not knowing the key was put into them as to hide it, not stopping you from taking the key nor stopping you from using the key to enter the hideout they are supposed to guard. They seem to enjoy trying to scare and dare monsters and use such manipulation for their own sake.
They are a humanoid skull on a wooden stick; a larger design than the magical staffs Woolly Blue Hoodoos carry with them. Unlike those objects, Skull Guard is capable of speech and choice. The top of their head is sliced off and emits smoke, supposedly from a candle of some sort placed within them.