Eothyris is only known from its skull, which is short and broad. With a total skull length of 5.7 cm (2.25 in), Benson et al. estimated the body to be 30 cm (12 in) long. The major distinguishing features of the skull are that it possessed a pair of long, large, fang-like teeth on each side of the upper jaw. The use for these teeth is not known, but Eothyris might have used them to eat small prey. All teeth in its mouth were sharp and pointed, with all but the enlarged four fangs being small and short. The skull is short and broad, two features which suggest that Eothyris had a snapping, rapid bite.
Eothyris is classified in the monophyletic family Eothyrididae. It is one of two genera in the family, the other being Oedaleops. The family is grouped in Caseasauria, and only cranial remains are known from it. The family is greatly supported, with nine dental and cranial features. Below is the cladogram of the analysis of Riez et al. (2009). Template:Clade