There is no such thing as a poisonous horse
No, black flies are not poisonous. Some may bite and leave marks as they were infected, but they are not poisonous
A horse that eats a poisonous plant may sicken, and he may die.
no
No, crane flies are not poisonous. They do not have a stinger nor mouthparts that can bite. Crane flies are completely harmless.
no
There are horse flies in Virginia, same as there are horse flies almost everywhere else in the world. There are specifically more horse flies in the east coast, especially in Florida. As for Virginia there's nothing special about the horse fly population there. Just the same old horse flies like everywhere.
Your horse cannot eat a poisonous plant.
It dies.
Horse flies don't like water because if they went in water it would kill them and horse flies prefer blood.
No, they are not. The nuts that are called chestnuts are poisonous to horses and ponies though.
Pteronarcophobia is the fear of flies, which includes horseflies.