Which pig writes the poem praising Napoleon?

You can find the answer to this question quite early on in Chapter 8.  The answer is that Minimus wrote the poem.  He is a pig who is seen as the best writer among the pigs on the farm.  The poem he writes in Chapter 8 is called Comrade Napoleon.

The poem is essentially a hymn to Napoleon.  By this point, all of the animals are being pushed toward adoring Napoleon as sort of a state religion, almost.  This is supposed to represent the way that Joseph Stalin was so admired and adored in the Soviet Union.