Final answer:
The cranial suture in horses typically becomes ossified as the horse matures, leading to a process known as synostosis where the adjacent cranial bones fuse together.
Step-by-step explanation:
Among the given options for joints in horses that typically become ossified as the horse matures, the cranial suture is the correct answer. Sutures are fibrous joints found only in the skull, and they are functionally classified as synarthrosis, meaning they are immobile or essentially immobile joints. In newborns and infants, the areas of connective tissue between the skull bones, known as fontanelles, allow for the flexibility and growth of the skull. As a horse matures, these fontanelles decrease in width and are replaced by narrow fibrous joints called sutures. Over time, the connective tissue can ossify, causing adjacent cranial bones to fuse to each other, a process known as synostosis.