I admit this may be a bit unconvincing. But the Gwynedd Archaeological Trust do list it as an 'alleged barrow'. So it may yet be there, and it may yet be something prehistoric.
Bach ab Carwed or Carwyd was the founder of Eglwys Fach [...] the parish is situated partly in Denbighshire and partly in Carnarvonshire [...] He is supposed to have been a Northern chieftain and warrior, who, retiring into North Wales, fixed upon this sequestered spot, and dedicated the close of his life to religion. [...]
Edward Lhuyd in his Itinerary of Wales (1698-9) says that Bach killed a certain wild beast which was the cause of much annoyance to the inhabitants on the banks of the Carrog near the church. The beast was a kind of wild boar, and they called it Carrog. A little after the slaughter Bach happened to kick the monster's head, but through contact with one of its tusks bruised his foot, and died of the wound (cf. the case of Diarmait in the Irish legend). Another version represents this monstrous boar, which played the part of a mediaeval dragon, as having been killed by the united action of the inhabitants. There is yet another tradition, which attributes its slaughter to S. Beuno, who paid Eglwys Fach a special visit for the purpose. According to this, Carrog somewhat resembled a flying serpent, which made its appearance in the daytime, kidnapping and eating children. S. Beuno, from the church tower, directed an arrow to the tender spot on its throat - the only vulnerable part on its body - and this took fatal effect. There is a tumulus, called Bedd Carrog, at Eglwys Fach, which tradition points out as the monster's grave. The word carrog means a brook or torrent and is the name of some half a dozen streams in Wales. A good number of the Welsh river names bear a "swine" signification, or are in some way or another associated by legend with swine.