I think that this is a very interesting manner of characterization. The sheep are linked to dashed hopes because they are so easily led. The sheep represent those masses that are so easily swayed. They do not voice any dissent and all bleat in unison whatever mantra Squealer teaches to them. They can be seen in the form of dashed hopes because they could represent so much in way of change. If all of them consolidated their efforts, thought for themselves, and fought for change, they could be a force with which the government of the pigs would have to listen. In the end, though, they are nothing more than the extension of the government, being the proverbial "yes- men" that prop up and support the government. The sheep represent dashed hopes because the tenets of Animalism that demand that animals think and act for themselves are undercut by the blind following that the sheep represent. In this, the sheep represent nothing more than an extension of the Status Quo, perfectly content with what is as opposed to what can be and or what could be. In this, the sheep represent a sense of dashed hopes.