Why did mares naturally evolve to foal in the spring?

Mares evolved to foal in the spring primarily due to environmental and resource availability. Spring provides abundant grass and forage, which are crucial for lactating mares and growing foals. Additionally, warmer temperatures during this season increase the chances of survival for the newborns. This timing aligns with the natural reproductive cycle of many herbivores, optimizing the chances of successful rearing in a favorable ecosystem.