You may worry if a weak horse is constantly being bullied into a tight place - a corner in the pastures, a shelter or alike - and being attacked with serious kicking and biting there. Following around, nipping - that's completely okay, boys will be boys. You shouldn't also worry about small bites, tears, minor injuries that come from these fights - what seems brutal to people is nothing to horses.
Now, if a horse tries displaying such a behavior towards a human, or whilst being ridden/lead past other horses, it is time for some educating. But, among horses - let them be. ;)
I have come a long way, to surrender my shadow to the shadow of my horse.