Later in the novel after Snowball has been expelled from Animal Farm, the animals are told that the medals awarded to Snowball in Chapter Four were a mistake and that they (the pigs) now know that Snowball fought on Jones' side in the Battle of the Cowshed. So, in his absence, his medals are taken away from him.
However, this is all lies - part of the propaganda about Snowball that Napoleon spreads through Squealer to discredit Snowball and strengthen his own position as leader. In fact, the animals are told several different versions of Snowball's behaviour at the battle - all of which are lies. This represents the way the authorities in the Soviet Union changed the past to make Stalin (Napoleon) look good and to make Snowball (Trotsky) look bad.
This section of the novel comes at the end of Chapter 4 and the description of "The Battle of the Cowshed." It described the first actual battle that the animals have fought as they fight off the humans that have invaded Animal Farm. Their great success has been due to Snowball's skills as a tactician, which have clearly saved the day. To commemorate this valiant victory, they create a military decoration, which is bestowed upon Snowball, and also decide to commemorate this day. Note what we are told about this:
The animals decided unanimously to create a military decoration, "Animal hero, First Class," which was conferred there and then on Snowball and Boxer. It consisted of a brass medal (they were really some old horse-brasses which had been found in the harness-room), to be worn on Sundays and holidays.
So, we see the society of Animal Farm creating its own system of recognising the important military contribution of certain individuals and also beginning to commemorate important historical events.