One of the carvings on the left hand side of the stone is a sort of half man half fish that has a faint ‘familiar’ look to it. (see photo)
The name of this, as I always understood it, was Darren Dusant meaning Black Saint (cliff or rock of), it’s not the spelling on the map but three saint would be tri-sant as in Llantrisant which is very near by.
On OS maps there is also “Well” on the left(West) side of the lane directly above it and Lle’r Gaer is next to them both
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It's not likely that Dusant would mean 'black saint', satisfying though that would be, because Welsh is an accretive language. This means that the adjective follows the noun, and, forming a compound word would likely undergo soft mutation. So Black Saint would be Santddu (or Sant du). Tarren Deusant might mean the cliff of two saints, and this would correlate with the 1696 note of two faces carved on the rock.
coflein.gov.uk/en/site/307746/details/TARREN+DEUSANT%2C+ROCK+CARVINGS+%26+POSSIBLE+HOLY+WELL%3BCELTIC+PAGAN+SHRINE/