Aram was surprised to see Mourad riding a beautiful white horse because they were too poor to afford a horse.

Explanation:
- Aram and Mourad were two young boys who lived in a small village in the mountains of Armenia. The village was poor, and the people had very limited resources.
- The first reason why Aram was surprised to see Mourad riding a beautiful white horse is that their family was too poor to afford such an expensive animal. Horses were considered a symbol of wealth and were usually owned by wealthy landowners or aristocrats.
- As mentioned in the story "The Summer of the Beautiful White Horse" by William Saroyan, Aram's family had a small farm and they struggled to make ends meet. They did not have the financial means to own a horse, let alone a beautiful white one.
- The sight of Mourad riding a horse was unexpected and unusual for Aram because it went against their family's economic status. It was a stark contrast to their simple and modest lifestyle.
- The story also portrays the boys as honest and hardworking individuals. They were not the type to steal or engage in dishonest activities. Therefore, the possibility of them stealing a horse to ride it was ruled out.
- Additionally, the story does not mention any fear or aversion towards horses. Aram and Mourad were familiar with animals and had been surrounded by farm animals all their lives. So, the reason for Aram's surprise cannot be attributed to a fear of horses.
- The final reason is that Aram mentions that he had never seen a horse before. This suggests that horses were not commonly seen in their village, further supporting the idea that their family's economic situation made it unlikely for them to own a horse.

In conclusion, Aram's surprise at seeing Mourad riding a beautiful white horse stems from the fact that their family was too poor to afford a horse. The sight of such an expensive and prestigious animal in their village was unexpected and surprising to Aram.