In Cornish folklore, what ugly fairy creatures guard ancient tombs?

In Cornish folklore, ugly fairy creatures said to guard ancient tombs are called Spriggans.

Spriggans are especially associated with West Penwith in Cornwall. Traditional descriptions portray them as grotesquely ugly, wizened old men with large heads, although their small forms could supposedly swell to enormous size. They were linked with old ruins, cairns and barrows, where they guarded buried treasure.

Folklore gave them a hostile reputation. Spriggans were blamed for whirlwinds, crop-damaging storms, theft, collapsing buildings and the kidnapping of children, sometimes leaving changelings behind. One story says a woman defeated them by turning her clothing inside out, a traditional method of repelling fairies.

They are easy to confuse with Cornish knockers and buccas, but those beings were associated with mines and could act as protectors of miners. Spriggans, by contrast, are chiefly remembered as dangerous guardians of ruins and treasure.