A number of round barrows known as 'The Beacons' sit on Mynydd Garthmaelwg. Marie Trevelyan recounts a peculiar story about this mountain:
The following story about a black snake was told in the first half of the nineteenth century. It must have been a very old story because the narrator always located it on the nearest mountain to his home and this particularly black reptile appeared to have no fixed abode. In Carmarthenshire it was located among the Van Mountains; in Pembrokeshire it was found in the Preceley Range; while in Glamorgan its home was the Great Garth, the Llantrisant, or Aberdare Ranges. The story ran thus: A great black snake was seen coiled in the sunshine. Its head and tail did not exactly meet, but left a small opening. In the middle of the coil there was a large heap of gold and silver and copper coins. A working man once saw all this treasure, and he resolved to have some for himself. There was nothing to be done but to just pass through the opening between the black snake's head and tail, and step in. At first the man was afraid, but, mustering up courage, he stepped in. He saw that the snake was asleep, and there would be no harm in having some of the coins for himself; so he began to fill his pockets with gold, silver, and copper. When his pockets were full, he took off his coat, laid it down, and began filling it with more treasure. Greediness made him forget the snake, but a fearful roaring frightened him. He immediately left his coat where it was, and fled. Looking back, he saw the black snake and the treasure sinking into the mountain, and the noise ceased.
Until a few generations ago, the Brenin Llwyd or Grey Monarch of the Mists was believed to inhabit this mountain and woe betide anyone caught in his grasp! A walk from Llanharan towards Llantrisant over the mountains will still take you to the site of "The Beacons", where before the 1700s the Militia met to muster and show arms. In later times this beacon would be lit to celebrate coronations. A short distance to the east is the location of the popular Egg Wells, whose sulphurous waters attracted hundreds of summer visitors to sample their curative properties and enjoy the fairground atmosphere.