In this work, we describe an anatomic anomaly of the sternothyroideus muscle for the first time. The dissection of the left side of the neck of a young adult female dog showed a group of fibres of the sternothyroideus muscle diverging cranially and ventrally from the caudal region of the muscle onwards to be inserted at the thyrohyoideum bone, close to the insertion site of the sternohyoideus muscle. Dissection of the right aspect of the neck revealed a normal right sternothyroideus muscle. The consequence of the presence of this muscle strip was a link between the sternothyroideus muscle and the apparatus hyoideus, and it can be interpreted as a sign of the close phylogenetic relationship between the sternothyroideus and the sternohyoideus muscles.