Who teaches the sheep to chant “Four legs good, two legs better” in Animal Farm?

The mantra or slogan of "Four legs good, two legs better"
is something seen in the last chapter of Orwell's work.  At this point in the novel, the
pigs have completely assumed full control of life in Animal Farm.  The revolution that
brought them into power is no longer something that is in the memory of most of the
animals.  Clover is one of the few holdovers, while most of the other animals consist of
pigs and dogs, the brains and the brawn of the farm's administration.  With no
foreseeable threat on the horizon, the pigs slowly assume all mannerisms of the humans
that used to control the farm.  One day, Squealer takes the sheep far away from the farm
to teach them a new song, for which he says "privacy is needed."  This coincides with
what Clover and the other animals see in a pig walking on its hind legs.  As the pigs
walk upright, mirroring the humans they once overthrew, Squealer recognizes that this
sight might trigger the former slogan of "four legs good, two legs bad."  It is why at
this moment, the sheep bleat out the new slogan of "four legs bad, two legs better" to
once again legitimize the rule of the pigs, something that Squealer mastered over the
course of the work.  The sheep bleat this on for so long that all potential signs of
dissent are muzzled, indicating the absolute nature of the pigs' rule on the
farm.