A single tree in an open area is an attractant for lightning, and a horse which shelters beneath a tree that gets zapped will likely not survive. Ornamental, toxic trees might be beautiful, but they can be deadly. Also, even if a tree can't kill a horse, a horse can kill a tree. Also, some trees like black walnut and red maple are highly toxic. Horses will browse the limbs, so you need to keep horses several feet back to prevent them from reaching the tree.
Ponds can be dangerous depending on the season, age of the horse, and what lies within.
Some ponds are riddled with debris such as broken glass, cans, wire, machinery parts, etc. Ponds in certain areas contain snapping turtles or alligators. Thin pond ice can present special problems. If a horse falls through the ice into deep water, it can drown or die of hypo-thermia. Mo. He notes that horses are more apt to walk on a frozen pond when it is covered with snow.