Background: Exercise is vital for horse health, improving fitness and strength, but it can also stress the body by producing harmful molecules that may damage cells. Young horses with mixed breeding may respond differently to exercise compared to purebred horses, yet little is known about how their blood and stress responses adapt to different exercise levels.

Objectives: This study explored how moderate and intense exercise affects blood health and stress markers in young horses with mixed breeding, aiming to understand if moderate exercise protects their health while intense exercise poses challenges.

Methods: Ten young horses (aged 3–4 years, four females, six males) followed a six-week moderate exercise program, ending with one intense exercise session. Blood samples were collected one hour after exercise on days 0, 14, 42, and 43 to measure blood cell counts (red and white cells, platelets) and stress markers (indicators of cell protection and damage). Data were analyzed to detect changes over time.

Results: Blood cell counts remained stable, showing the horses’ ability to handle exercise stress. Moderate exercise improved protective markers and reduced damage indicators, suggesting better cell health. However, the intense session increased stress markers, indicating temporary cell strain.

Conclusions: Moderate exercise benefits young mixed-breed horses by enhancing cell protection, while intense exercise may cause stress that requires recovery. Tailored exercise plans can optimize horse health and performance.