Gorynychus is a genus of basal therocephalian from the Middle to Late Permian from Kotelnich, Russia. It was named after the three-headed dragon Zmey Gorynych (Змей Горыныч) from Russian mythology. The genus contains two species, G. masyutinae and G. sundyrensis. G. sundyrensis, along with Julognathus, were the only known large carnivores present in the Sundry Assemblage, replacing the anteosaurs, which went extinct following the transition from the Isheevo Assemblage Zone to Sundry Assemblage. With a skull length of 35–40 cm (14–16 in), it was one of the largest therocephalians.

The genus would survive the Capitanian mass extinction event, with G. masyuntinae, the type species, being found in the early Late Permian rocks of the Kotelnich Tetrapod Assemblage. Despite being notably smaller than G. sundyrensis, it was the top predator of the assemblage with only Viatkogorgon and Viatkosuchus approaching its size.

Kammer and Masyutin (2018) recovered Gorynychus as a basal therocephalian just outside of Eutherocephalia. On the other hand, Suchkova and Golubev (2019) recovered Gorynychus as a lycosuchid, following the discovery of G. sundyrensis. Liu and Abdala, in their 2019 study, recovered Gorynychus as a sister taxon to Lycosuchus. However, in a later study, they recovered it intermediately between Lycosuchus and Scylacosauria.

Cladogram recovered by Liu & Abdala (2019):

Cladogram recovered by Liu & Abdala (2023):

G. masyutinae, only known from its holotype, was wolf-sized, with a skull length of 20 cm (7.9 in). Like many theriodonts, it had strongly developed and prominent canine teeth. The postcanine of Gorynychus was unique among therocephalians, being labiolingually compressed and were "leaf" or "spade" shaped, with proportionally long serrations on both mesial and distal edges. G. sundyrensis was estimated to have a skull length of 35–40 cm (14–16 in), making it one of the largest known therocephalians. It had four postcanine teeth compared to G. masyutinae, which had three. The premaxilla was low and relatively short and contained four incisors. The teeth of G. sundyrensis were noted to have been serrated.

Within the Sundyr Assemblage fauna, G. sundyrensis coexisted with the more common Julognathus, with both therocephalians coexisting by niche partitioning, with Gorynychus occupying a scavenger niche based lifetime wears of the tooth crowns. Large herbivores are poorly known, only represented by dinocephalians possibly similar to Ulemosaurus, while small herbivores were represented by the anomodont Parasuminia. A number of other tetrapods are also known from this assemblage, including Leptoropha aff. talonophora, Microphon, Enosuchus sp., and the chroniosuchian Suchonica, as well as the temnospondyl amphibian Dvinosaurus gubini.

The Sundry Assemblage was the terminal stage of the Dinocephalian tetrapod fauna before the Capitanian mass extinction, representing a crisis stage in the development of the tetrapod community in Eastern Europe and was transition from the Dinocephalian fauna to the Theriodontian fauna of the Late Permian. The presence of dinocephalians suggests the assemblage correlated with the uppermost subzone of Tapinocephalus Assemblage Zone. Compared to the older Isheevo Assemblage Zone, the aquatic fauna was more similar to the Theriodontian faunas, with the chroniosuchids, karpinskiosaurids, and kotlassids appearing and becoming abundant. On the other hand, the last archegosauroids went extinct. Within the terrestrial fauna, anteosaurs went extinct and were replaced by large therocephalians.