What does 'after their fashion' mean?
A fragment from Animal Farm:

Sometimes it seemed as though the farm had grown richer without making the animals themselves any richer, except, of course, for the pigs and dogs. Perhaps this was partly because there were so many pigs and so many dogs. It was no that these creatures did not work, after their fashion. There was, as Squealer was never tired of explaining, endless work in the supervision and organization of the farm.

Oxford dictionary says, that:
after a fashion
to a certain extent but not perfectly

after (or in) the fashion of
in a manner similar to

I guess the second meaning is used in the text but who're refererred to by they - after THEIR fashion? It could be people, because they used to behave this way before the Rebellion, but nothing is said about them in the next sentences.

Question

What does 'after their fashion' mean?
A fragment from Animal Farm:

Sometimes it seemed as though the farm had grown richer without making the animals themselves any richer, except, of course, for the pigs and dogs. Perhaps this was partly because there were so many pigs and so many dogs. It was no that these creatures did not work, after their fashion. There was, as Squealer was never tired of explaining, endless work in the supervision and organization of the farm.

Oxford dictionary says, that:
after a fashion
   to a certain extent but not perfectly

after (or in) the fashion of
   in a manner similar to

I guess the second meaning is used in the text but who're refererred to by they - after THEIR fashion? It could be people, because they used to behave this way before the Rebellion, but nothing is said about them in the next sentences.

Answer

It's referring to the pigs and dogs. The other animals do work, probably physical labor(it has been many years since I've read Animal Farm), but don't become any richer, while the pigs and dogs supervise and do become richer. So the pigs and dogs work, after their fashion, or in their own way, but it's not the same type of work that the other animals do. I think Squealer is trying to defend the type of work he does as difficult and time-consuming to justify the fact that pigs and dogs earn more for their work.

Answer

Alexander, you're right it's the second meaning. 'Their' fashion refers to 'these creatures' earlier in the same sentence, which in it's turn refers to 'pigs and dogs' in the previous sentence. Whether or not it refers to people is up to each individual reader to interpret.

Answer

Animals work "after their fashion".

I think this kind of expression comes from the bible. God mad all creatures "according to their kind/fashion." 
It follows that all creatures do "after their fashion." Beavers make dams. Birds build nests. An so on.