Chaetostoma formosae differs from all the species currently in Chaetostoma with the exception of Chaetostoma anale and Chaetostoma jegui by the presence in mature males of an enlarged second unbranched anal-fin ray bearing two distinct posterior dermal folds, and reaching the base of the caudal fin when fully developed. Chaetostoma formosae differs from C. anale in having an angular distal pelvic-fin margin in mature males (vs. W-shaped distal margin in mature C. anale males and the leading pelvic-fin ray neither elongate nor filamentous, not reaching beyond the adjacent branched ray in mature males (vs. leading pelvic-fin ray both enlarged and filamentous, longer than the remaining rays in mature males of C. anale). Chaetostoma formosae differs from C. jegui in having dark spots restricted to the head and the dorsal region surrounding the dorsal-fin base, and with compound pterotic and lateral plates bordered by dark pigment (vs. dark background with light blotches in C. jegui); and by the shape of the distal margin of the pelvic fin strongly projected and angular in mature males, vs. distal margin straight and with leading-fin ray sometimes surpassing the distal margin of the fin as evident in photographs) (Ref. 88853).