Hisonotus thayeri can be distinguished from H. aky, H. brunneus, H. carreiro, H. charrua, H. heterogaster, H. laevior, H. megaloplax, H. montanus, H. nigricauda, H. notopagos, H. ringueleti, H. taimensis, and H. vireo by having the anterior portion of snout completely covered by odontodes (vs. anterior portion of snout with an odontode-free band between the dorsal and ventral series of odontodes). It can be diagnosed from H. alberti, H. depressicauda, H. depressinotus, H. francirochai, H. maculipinnis, H. packysarkos, H. paulinus, and H. prata by having the anterior portion of abdomen covered by large plates arranged in three longitudinal series without naked spaces between them (vs. abdominal plates , when present, not arranged in three longitudinal series, or when arranged with naked spaces between the lateral and median series). It can be differentiated from H. acuen, H. bockmanni, H. chromodontus, and H. vespuccii by having an ovoid to rectangular spinelet (vs. spinelet V-shaped, with lateral projections anteriorly directed). It is distinguished from H. iota and H. leucophrys by the lack of well-developed odontodes at the posterior tip of supraoccipital (vs. presence). It is distinct from H. armatus and H. hungy by having 28 vertebrae (vs. 25-26 in H. hungy and 29 in H. armatus). It is distinguished from H. leucofrenatus by having the clear longitudinal stripe of head ending near the vertical through distal tip of opercle (vs. clear longitudinal stripe longer, bifurcating at the tip of compound pterotic and running parallel to each other; the inferior stripe over the canals of the lateral line, reaching the vertical through the middle of dorsal-fin base); bifid neural spine of seventh vertebra present, dorsolaterally projected (vs. bifid neural spine absent); and basipterygia contacting along midline until anterior margin (vs. anterior portion of basipterygia with halves apart from each other). It differs from H. notatus by its generally continuous mid-dorsal series of plates, with a total of 15-18 plates (mode 17), rarely with one plate lacking in the middle of the series (vs. mid-dorsal series generally interrupted, with 4-13 plates present, and 5-8 lacking plates in the middle of the series) (Ref. 109747).

Description: Dorsal-fin branched rays 6-7; anal-fin unbranched rays 4-5; pectoral-fin branched rays 5-6; pelvic-fin unbranched rays 5-6 (Ref. 109747).