Benjamin, the stubborn donkey, is the only animal who does not believe in the positive prospects of the windmill.

An older animal, Benjamin is rather cynical because he feels that no matter what happens, his life will not get any easier with the windmill. Nor does he feel that it is worth his while to become involved in the new movement: "Benjamin was the only animal who did not side with any faction." He does not raise criticisms of the animals' neglect that he has noticed because he does not want to bring trouble upon himself.

One interpretation of Animal Farm suggests Benjamin may represent the Jewish population of Russia; made wary by previous abuses and suspicious of the new regime, he is in a position to recognize the dangers of the pigs' rise to power. Benjamin might also be regarded as a representation of the cynical intellectuals who recognized the evils of the new Russian regime but, believing corruption inherent in any system, ultimately failed to put forward any strong opposition to it.

I think that the answer to this is Benjamin, the donkey.  You can find this answer about midway through Chapter 5.

Benjamin is a very cynical animal.  He believes that things are going to be bad pretty much no matter what.  This attitude can be seen in what he thinks about the windmill.

Snowball says the windmill will make the animals' lives easier.  Napoleon says it will be better to have no windmill because then the animals will have more food.  Benjamin is cynical about both claims.  He does not think life will get easier with the windmill and he also does not think there will be more food if they don't build it.