A male horse that has not beencastrated is known as a stallion.
I am actually really fond of these animals; I find them to be really graceful. A stallion is usually not an aggressive animal though that obviously depends on the kind of training. They require very careful management if you want perfection. The term stallion actually dates back to the era of Henry VII.
They are herd animals but can be trained to take parts in horse shows and horse racing etc.