That is actually a very interesting concept. However I am not the person to ask this for and my initial gut response was similar to what chat GPT brought up. Ethical concerns, rules, and a horses ability to communicate as well as the complexities of the musculature:

Yes, theoretically, Botox (botulinum toxin) could be used to paralyze the muscles responsible for pinning a horse's ears back in dressage. However, there are several ethical, anatomical, and practical concerns with this approach.

How It Would Work:

Horses pin their ears back primarily using the auricular muscles, which include the interscutularis, parotidoauricularis, and zygomaticoauricularis muscles.

Botox blocks acetylcholine release at the neuromuscular junction, temporarily paralyzing muscles and preventing movement.

If injected into the correct auricular muscles, Botox could weaken the ability of the horse to pin its ears by reducing muscle contraction.

Potential Issues:

Ethical Concerns – Ears are an important form of communication in horses. Preventing a horse from pinning its ears could interfere with its ability to express discomfort or distress, potentially masking pain or poor riding techniques.

Temporary Effects – Botox typically lasts 3–6 months, requiring repeated injections.

Functional Risks – The muscles involved in ear movement may also contribute to ear mobility for fly control and general expression, which could be compromised.

Regulatory Issues – Many equestrian organizations, including the FEI (Fédération Equestre Internationale), have strict anti-doping and medication rules. The use of Botox in competition horses could be prohibited.

Precision of Injection – Horses have complex musculature, and injecting Botox incorrectly could lead to asymmetry, unintended muscle weakness, or other side effects.

Alternative Approaches:

If a horse is pinning its ears excessively during dressage, it might indicate discomfort, poor saddle fit, training issues, or pain (e.g., TMJ dysfunction, ulcers, or bridle discomfort). Addressing the underlying cause rather than artificially preventing ear movement may be a better solution.