According to the New Statistical Account for Fortingal (1845, v10):
Caisteal coin a-bhacain- the Castle of the dog's kennel.
This bacan, or stake to which the Fingalians tied their stag-hounds, and from which the castle is named, is a thin stone, about 2 and a half feet in height, resembling the letter q, with the small end set into the ground, up on a little green eminence.
It is known as Caisteal a' Chonbhacain, from a remarkable stone in its vicinity, which was till recent times practically an idol.
This stone is also associated with the legends of the Fianna, led by Finn MacCool / Fionn MacCumhaill / Fingal. It is said that Finn used to tie his dog, Bran, to this stone while he was at his nearby fort, hence it's shape, and it's name Bhacain, or tether-stake.
Archie McKerracher wrote of a legend that this stone was "used to protect maidens' virtue, for when the girls returned from harvesting in the godless Lowlands they were sent underneath it, and no unwanted children occurred"