Materials that affect normal biochemical processes in the body are toxins (plant or animal origins such as venom or bacteria) or poisons (any substance that causes illness, teratogenic birth defects or death). Each topic addresses one of these. Thankfully, few substances overtly affect our domesticated horses, but the results are usually fatal when they do. These include animals (snake bites), bacteria (tetanus) and plants (yew, red maple, black wall nut shavings). Some can be transient such as the seasonal clover drools. Some can cause abortion of the fetus, such as ergot mold. Some cause death.
More concerning are the toxins and poisons that are subtle in their effects. We don’t normally call them poisons, but they drain the horse of vitality over time or cause their premature death. These include heavy metals in the soil or feeding inflammatory food over the years.
Avoidance is the key because, with toxins and poisons, treatments often have poor results.