Both Early Irish and Russian mythological traditions demonstrate a particular example of an extraordinary character showing supernatural features as well as the features of a chthonic monster: it is Cú Roí mac Daire on the Irish side, and Svyatogor on the Russian side. We have to be careful before arguing that these two mythological characters reflect one particular archetype of a monstrous chthonic creature (cf. views expressed by Henderson (1899) in Ireland and Putilov (1986) in Russia); on the contrary, one has to consider both heroes as complex and independent entities who appear in the two quite distinct mythologies (Early Irish and Russian). This is especially true in relation to the Russian tradition of byliny (былины) which have been preserved orally until the first published editions of the nineteenth century. Cú Roí and Svyatogor, the two mythological characters discussed, play essentially the same rôle of chthonic monsters in the basic myth. They act as an ‘obstacle’, ‘barrier’ for human heroes such as Ilya and Cú Chulainn. They are primeval characters in both traditions, that is why they are not associated with the dominant population groups: Svyatogor is not of Rus’ but from outer space (mountains on the borderland, Carpathians?), Cú Roí is from outer Munster, from marginal auto-chthonous (sic!) population groups. At the same time both characters as they have survived in the literature are contaminated by Biblical and apocryphal stories of Samson and Delilah. This is how they became incorporated into a comparatively new synthetic literary tradition.