Luckily, tomato's fruits and mature potatoes aren't toxic to us. But, the green parts of these and many related plants of this family contain atropine or atropine-like substances. Atropine will slow the gut. That's the last thing a horse wants because it will cause colic, which can kill a horse. Other toxic members of this family include ground cherry, henbane, and jimsonweed.
Some plants contain saponins, which are detergents, or soaps, and present another danger for horses.
Plants that contain saponins include pokeweed, bouncing bet, corn cockle, and English ivy. Though members of different plant families, they all kill liver cells. When plant toxins damage the liver, photoactive substances derived from chlorophyll build up in the blood. These substances are exposed to the sun only in the white areas, so that's where you'll notice blistering, cracking, or loss of outer layers of the skin. Japanese yew can stop the heart suddenly.