According to the newly-added Horses section on the Official Site, Aragorn rides a horse called Brego, named after the son of Eorl and second King of Rohan, in The Two Towers. However in the novel itself, Aragorn is presented with a horse called Hasufel.
Does this mean that Peter Jackson has made an unnescessary change to the original story by changing Hasufel’s name to Brego? Maybe, but I believe that this indicates that Aragorn rides not one but two horses in The Two Towers. This theory is supported by (spoilers – highlight to read) extracts from a copy of The Two Towers script leaked on the Internet which describe Aragorn being left for dead on the plains of Rohan and watching the huge army of orcs marching towards Helm’s Deep – the number of which he describes to Théoden in the latest TTT trailer. It is very likely that Aragorn’s horse is killed in a skirmish, thus leaving him stranded. .