Why is the English FA Cup final of 1923 also known as the "White Horse Final"?

The 1923 FA Cup between the Bolton Warders and West Ham United had a massive crowd attending. While officially there were 126,000 tickets sold, the actual attendance was much higher. The anticipation for the game and the desire for seats led to fans covering the pitch making it impossible to start the game. In comes "Billie" the white horse of a policeman who was trying to clear the pitch. The photo went on to be a symbol of the chaos. The House of Commons introduced legislation to help improve crowd safety after this event.

https://en.wikipedia.org/wiki/1923_FA_Cup_final

This video gives you an idea of the crowd at Wembley:

https://www.youtube.com/watch?v=c4NqYsFTr1Q