Question

What is the most likely reason that horses and mountain goats have hooves?

Answer 1

The most likely reason is that this is their adaptation to their environment. Because mountain goats and horses are required to run and walk on rugged terrain, they obviously need a bottom surface on their feet which is very rugged and doesn't break easily - this is what hooves provide them with. 

Answer 2

Homologous structures are those which are similar in origin but dissimilar in function. The organs are made of the same bone pattern and structure. Origin of hooves in both horse and mountain goat have same structure but performing different functions as they live in different habitats. The homologous structures represent divergent evolution.