Abstract The m. rhomboideus of domestic dogs typically originates from the dorsal median raphe of the neck and cranial thorax, with a distinct slip from the occiput, and is composed of the three parts: cervicis, thoracis and capitis. Here, an additional muscular slip was identified on the left side of an adult male cross-breed dog cadaver. This ribbon-shaped slip arose indirectly via tendinous fibres from the transverse process of the second cervical vertebra and extended caudodorsally to join the capital part near its scapular insertion. The slip was innervated by ventral branches of the cervical spinal nerves, consistent with the usual supply of the rhomboideus complex and distinct from the neighbouring m. serratus ventralis cervicis, despite their segmental overlap. Such a variant of the m. rhomboideus has not been previously reported in dogs. Recognition of incidental muscular variations, particularly with regard to their innervation patterns, provides insights into embryologic development and phylogenetic relationships in carnivores.

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