This scenario is likely an example of "cultural diffusion" or "cultural convergence." Cultural diffusion refers to the spread of cultural elements, such as styles, practices, or technologies, from one culture to another through contact and interaction. Cultural convergence, on the other hand, refers to the phenomenon where two distinct cultures develop similar traits or characteristics over time, often due to shared influences or environmental factors.
In this case, the similar style in horse sculptures between the two neighboring cultures could arise from direct influence (cultural diffusion) or from similar needs and circumstances leading to convergent evolution of artistic styles.