A Trolloc is a shadowspawn first created by Ishmael many centuries ago from his laboratories in the blight. Trollocs are not bred, they are engineered. They cannot mate. New batches are created as often as they are needed in the laboratories. Lews Therin Telamon considers the destruction of these laboratories one of the most necessary objectives for the people of the dragon to accomplish if they are to win the war of the ages.
Trollocs were a mistake in a particular experiment, they are natural hunters but lack of game in the blight caused them to pour out from the blight into central Ghillodahn in search of food. The Dark One sought to harness their power and ferocity and used them to initiate the war of the ages.