I think this is one of the continuity errors in the films so we won't see them getting the horses back. are you sure they're the same ones? does aragorn call his 'new' horse brego again?
Change the 'plot' of the book and you'll have problems. in the book, aragorn forces (and legolas coerces) the horses into following them. In the film, to stop the guys riding on horse back so they could travel through the caverns on foot, Jackson changes part of the story, and Hey Presto! we have another 'error'.
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