Looking for mythological characters similar to Cerberus.
Gargoyle - is a fantasy and horror monster inspired by the gargoyle architectural element. While they were believed in mythology to frighten away evil spirits, the idea of such statues physically coming to life was a more recent notion. Like golems, they are usually made of magically animated or transformed stone, but have animal or chimera traits, and are often guardians of a place such as a cathedral or castle. They can also be depicted as vessels for demonic possession, or as a living species resembling statues.
Golem - is an animated anthropomorphic being that is magically created entirely from inanimate matter (usually clay or mud). The word was used to mean an amorphous, unformed material in Psalms and medieval writing.
If you look only for animate living beings, as example:
Minotaur - is a mythical creature portrayed in Classical times with the head and tail of a bull and the body of a man or, as described by Roman poet Ovid, a being "part man and part bull". He dwelt at the center of the Labyrinth, which was an elaborate maze-like construction designed by the architect Daedalus and his son Icarus, on the command of King Minos of Crete.
Griffins and lamassu were the first creatures that popped into my head. They are both widely depicted as guardians of treasure or powerful figures.
Lamassu, however, are not explicitly guardians of something secret. They are protectors of the palace, kings, queens, royalty etc. and are thought to ward off evil. Although, they could be interpreted or adapted as guardians of secret palaces, temples, or burial sites.
And, of course, dragons. Famous dragons such as Fafnir and Smaug in The Lord of The Rings are notorious for hoarding and guarding heaps of glittering treasure and secret artifacts.
Hope this helped, have a nice day.
In Slavic Folklore, Baba Yaga often depicted as an old woman is a keeper of the land of the dead.
The Border Guard of the Dead-Lands
Baba Yaga is the customs officer of the land of the dead. Her hut is a kind of passport control. In that hut, the hero goes through all the necessary rites to begin his journey into the supernatural world. He washes in the sauna, which symbolizes the washing of a dead body before burial. He eats and drinks in excess, which is a hint at the traditional feast (pominki) held after a funeral. He always sleeps in the strange hut, because night is the witching hour, the time of passage from one state to another. And sleep is a kind of small death.
Here are more details about her, since she's not so well known:
The Hut on Chicken Feet
In the slumbering forest, in the deepest darkness, a hut stands on two chicken feet. In the hut, on the stove, Baba Yaga reclines on the ninth brick. Her legs are bony, her nose has grown all the way to the roof, and she’s sharpening her own teeth. She looks like an old woman, but there something odd about her. Who is she? Is she even an old woman at all?
Baba Yaga is an ambivalent character, and that makes her very interesting. She’s definitely scary—her face covered in warts, her crooked nose reaching all the way down to her hairy chin. She has hands like hooks, a hump on her back. Still, you can’t exactly call her a villain. Yes, she constantly threatens to eat Ivan the Prince, or Mashenka, or Vasilissa the Beautiful, but she ever actually eats anyone. Really, her purpose in the stories is to be a magical helper to the hero, even if her help is not always what’s expected.
Also, remember that her hut always face the land of the dead, you have to call it to turn around if you want to meet Baba Yaga and hope for a crossing right to the dead lands.