What is on the green flag and what does it symbolize?

From the text:

"Snowball had found in the harness-room an old green tablecloth of Mrs. Jones's and had painted on it a hoof and a horn in white. This was run up the flagstaff in the farmhouse garden every Sunday morning. The flag was green, Snowball explained, to represent the green fields of England, while the hoof and horn signified the future Republic of the Animals which would arise when the human race had been finally overthrown."