During the dissection and skeletal examination of 151 horses, a congenital malformation (CM) of the first sternal rib that influenced the aperture of the Thoracic inlet was noted in six horses. The presentation of this CM was variable between horses in gross anatomic appearance; notably, an absent first sternal rib, bifid tuberculum costae, bifid sternochondral articulation onto the sternum, flared shaft, normal first sternal rib inserting onto the cranial branch of a bifid sternochondral articulating second sternal rib, straight costal shaft, and an articulating rudimentary tuberculum costae with a ligamentous extension replacing the bony shaft and attaching to a rudimentary sternochondral articulation onto the sternum. Of the 151 horses examined, the CM of the first sternal rib was restricted to 6/60 Thoroughbred horses, and only in those that were affected by either the unilateral or bilateral transposition of the caudal ventral tubercle from C6 onto the ventral surface of C7. The normal anatomic presentation of the thoracic inlet was altered, along with associative musculature including neurological pathways. These CMs are likely to produce clinical and functional ramifications of the thoracic inlet, thoracic limb, and thoracic viscera, with the probability of altering postural and locomotive function as noted in four horses demonstrating the CM.