An interesting science project related to horses could be related to the emotional connection between humans and horses. For example, you could research the impressive results of equine-assisted psychotherapy, an approach to mental health counseling that uses horses as empathetic helpers in the therapeutic process.
Humans and horses have lived and worked together for thousands of years. Surprisingly, the topic of the cognitive and emotional dynamics between horse and human is relatively under researched. Even a small study on the emotional relationships between humans and horses could be very interesting and possibly of value to the academic community as a whole.