Actually, Mastiffs were bred for soldiers back in the Molosser days.
What is Mastiff?
A Mastiff is a giant breed of dog, descended from the ancient Alaunt and Molosser. The general term 'mastiff' is used to refer to many different breeds around the world, probably all descended from the same root stock. The proper noun 'Mastiff', however, is used to refer to a specific breed of mastiff, the breed that originated in England over 2,000 years ago.Breeders today have bred the Mastiff for gentleness and have created an excellent companion, large enough to deter intruders and yet gentle enough to be dependable around children.
My dog is the coolest dog I have ever had. I have had german shepards, huskey's, and a Akita. He is the kindess dog you will ever meet, but very protective. He doesn't like agression. Infact he gets in between my kids when they are playing agressively. Most of the time, he's pretty much afraid of his own shadow. Of course he's still young. He likes being with people. He never leaves the yard. I could go on and on. I can't say enough about him. Can you tell???
In this day and age, I've never heard of them("the breed") being attackers of any kind. There are all different offsprings of the Mastiff breeds. Ex. Bull Mastiffs(bull and Mastiff) Neopolitan(I've heard can be agressive), Cane Corso Masttiff (Spanish decendants). In all my research on the Mastiff history, I've never read that they bred to kill horses. If that were true, I wouldn't have gotten a mastiff. I love my horse and wouldn't put him in a bad situation.