Diagnosis: Petrochromis horii differs from its congeners with no overlaps in several morphological characters: its body is deeper than that of Petrochromis macrognathus; the caudal peduncle is deeper than that of P. orthognathus; the pectoral fin is longer than that of P. macrognathus; the pelvic fin is longer than that of P. fasciolatus; the caudal fin is longer than that of P. famula, P. fasciolatus and P. orthognathus; the interorbital region is wider than that of P. macrognathus and P. orthognathus; the upper jaw protrudes more than that of P. fasciolatus and less than that of P. macrognathus, P. polyodon and P. trewavasae; the soft rays in the dorsal fin are fewer than in P. macrognathus; and the number of scales in longitudinal line is larger than in P. famula and P. orthognathus (Ref. 95531).
Description: Body short, deepest at dorsal-fin origin, gradually becoming shallow to caudal peduncle; caudal peduncle short, depth of caudal peduncle 82.2-101.6% of its length (Ref. 95531). Dorsal profile of head sloping steeply; ventral profile slightly rounded (Ref. 95531). Eye small, rounded; eye-length 62.2-72.8% of snout length; interorbital region convex (Ref. 95531). Mouth lower than body axis; jaws almost equal anteriorly; posterior edge of mouth not reaching vertical line through anterior margin of eye (Ref. 95531). Oral teeth tricuspid, arranged in 9 to 11 irregular rows at anterior parts of upper and lower jaws, making brush-like, broad tooth plate on each jaw; outer and inner teeth indistinguishable in size, shape, or arrangement pattern (Ref. 95531). Gill rakers short (Ref. 95531). Lower pharyngeal jaw sub-triangular with concave lateral margins and roundly convex caudal margins on both sides; teeth villiform (Ref. 95531). Dorsal fin originating slightly anterior to vertical line through posterior margin of operculum; each spine with a lappet near tip; all soft rays branched; posterior tip of the fin elongate, extending beyond caudal-fin base when depressed (Ref. 95531). Anal fin originating slightly anterior to vertical line through base of last dorsal-fin spine; three spines increasing in length posteriorly; first and second spines with a lappet near tip; all soft rays branched; posterior tip elongate, extending beyond caudal-fin base when depressed (Ref. 95531). Upper end of pectoral-fin base slightly posterior to vertical line through posterior margin of operculum; distal tip acutely pointed; fifth soft ray from upper longest, reaching vertical line through anus; soft rays branched, except uppermost two and lowermost two (Ref. 95531). Pelvic fin originating posterior to vertical line through posterior margin of operculum; all soft rays branched; outer soft rays longer than inner ones; outermost soft ray filamentous, reaching anal-fin origin (Ref. 95531). Caudal fin lunate; distal tips of upper and lower lobes acutely pointed (Ref. 95531). Scales on flank cycloid or ctenoid with radially directed short ctenii; granulation extending over almost entire exposed part of scale; posterior half of cheek scaled; caudal fin with scales on fin membrane between rays; other fins lack scales; upper lateral line originating from sensory canal of supracleithrum, running below dorsal margin of body, posteriorly not reaching posterior end of dorsal-fin base; lower lateral line running on body axis, anteriorly not reaching vertical line through anal-fin origin, posteriorly extending beyond caudal-fin base (Ref. 95531).
Colouration: A part of head anterior to the posterior margin of eye moss green; pale ventrally; body with eleven moss green vertical bands from nape to caudal-fin base, and a moss green or dark green horizontal broad band on body axis; spaces between the bands and ventral part of body pale green (Ref. 95531). Fins moss green; anterior margin of pelvic fin pale green (Ref. 95531). The vertical and horizontal bands are unclear in fixed specimens (Ref. 95531).