Lamontichthys avacanoeiro differs from all congeners in the shorter first branched dorsal-fin ray (22.6-26.0%, vs. 26.4-40.5% SL) in specimens larger than 70 mm SL.

Lamontichthys avacanoeiro can be further distinguished from L. filamentosus, L. llanero, and L. maracaibero by the anterior tip of the snout with an oval shaped area without plates (vs. tip of snout totally covered with plates or, with only a very small area lacking plates), and by the teeth with long cusps (vs. short). Lamontichthys avacanoeiro can be further distinguished from L. filamentosus and L. llanero by the lower lip semi-oval shaped (vs. semicircular). Lamontichthys avacanoeiro also differs from L. maracaibero in having more than one plate between the base of the last pelvic-fin ray and the anterior margin of the anus (vs. one plate) and in the greater abdominal length in specimens larger than 150 mm SL (17.6-17.7% [n = 2], vs. 14.1-16.3% SL [n = 5]). Lamontichthys avacanoeiro can be distinguished from L. parakana by the more posterior location of the anal-fin origin (at the vertical through the distal tip of the last dorsal-fin ray, vs. anterior to that point), by the shorter unbranched anal-fin ray (16.9-20.3%, vs. 21.4-24.8% SL), and by the smaller orbital diameter (13.6-16.2%, vs. 16.6-17.3% HL) in specimens larger than 70 mm SL. Lamontichthys avacanoeiro also differs from specimens of L. parakana larger than 100 mm SL in the narrower head (91.3-100.9%, vs. 101.3-106.1% HL) (Ref. 83332).