Herbert Weir Smyth, A Greek Grammar for Colleges

a. Both stems have a common nominative singular: σκότος darkness, σκότου σκότῳ, etc. (like ἵππου ἵππῳ) or σκότους σκότει (like γένους γένει). So τὸν Ἄθω, and τὸν Ἄθων from Ἄθως (238 d), τὸν Σωκράτη and τὸν Σωκράτην (264 b). These are called heteroclites (ἑτερόκλιτα differently declined).

N. Many compound proper names in -ης (especially names of foreigners) have forms of the 1 and 3 decl., as Τισσαφέρνης, -νους, -νῃ and -νει. So Θεοκρί_νη (voc.) in Demosth., Λεωνίδην and Λεωνίδεα in Hdt.

b. Certain cases are formed from another stem than that of the nom. singular: ὁ ὄνειρο-ς dream, gen. ὀνείρατ-ος (as if from τὸ ὄνειραρ), or (less freq.) ὀνείρου; so τὸν Ἀπόλλωνα and τὸν Ἀπόλλω (260), τοῦ υἱέος and τοῦ υἱοῦ (285, 27). These are called metaplastic forms (μεταπλασμός change of formation).