Doyle's grandmother (or mother) was a Hellhound of the wild hunt. She bred with Doyle's grandfather (or father), a phouka in his dog form.
Queen Andais in A Caress of Twilight claims it was Doyle's grandmother. Doyle also speaks of her as his grandmother.
Hettie in Swallowing Darkness says it was Doyle's mother. Hettie is more likely to be wrong than Doyle himself.