Congenital myotonia in horses is a rare, inherited neuromuscular disorder that affects how muscles relax after contraction. Instead of returning quickly to a resting state, affected muscles remain contracted for longer than normal, resulting in temporary stiffness that is often most noticeable after periods of rest. This condition has also been described in other species, including humans and "fainting goats," where delayed muscle relaxation leads to brief episodes of rigidity. In horses, stiffness typically improves with movement, but it can still affect comfort, coordination, and overall performance.

Autoimmune disease in horses occurs when the immune system mistakenly attacks the body's own tissues instead of protecting against infection. Rather than targeting bacteria, viruses, or parasites, the immune response becomes misdirected, leading to inflammation and tissue damage. Although autoimmune diseases are relatively uncommon in horses, they are often chronic and may significantly affect long-term health. Conditions such as equine recurrent uveitis, pemphigus foliaceus, and rare immune-mediated blood disorders illustrate how immune dysfunction can impact the eyes, skin, or circulatory system. Because autoimmune diseases can affect multiple organ systems and often resemble infections or other inflammatory conditions, diagnosis can be complex. Many cases require careful evaluation, laboratory testing, and exclusion of other causes before an autoimmune origin is confirmed.

Rat poisoning in horses occurs when horses accidentally ingest rodenticides, which are pesticides used to kill rats and other rodents. Exposure can happen through contaminated feed, water, pasture, or bedding, especially in barns, tack rooms, and feed storage areas where rat poison is used. Different types of rat poison affect the body in different ways. Some rodenticides interfere with normal blood clotting and can cause internal bleeding, while others attack the nervous system or disrupt cellular metabolism, leading to rapid and severe toxicity.

Nightshade poisoning occurs when horses ingest plants from the genus Solanum, a group that includes several species that produce toxic compounds. Plants such as black nightshade (Solanum nigrum), bittersweet nightshade (Solanum dulcamara), and horse nettle (Solanum carolinense) contain naturally occurring toxins that can affect the gastrointestinal tract, nervous system, and cardiovascular system. Many nightshade species grow as weeds in agricultural environments, including pasture edges, disturbed soils, roadsides, and hay fields. Although horses often avoid these plants under normal grazing conditions, exposure can occur when toxic species contaminate hay, grow in overgrazed pastures, or become accessible through agricultural waste such as potato plants or cull potatoes. Nightshade poisoning is relatively uncommon in horses, but ingestion of certain species can lead to digestive irritation, neurological abnormalities, and other systemic effects. Understanding which plants belong to the nightshade group, how toxicity occurs, and how to prevent exposure helps owners and caretakers recognize potential risks and respond quickly if poisoning is suspected.