The stallion was the emblem of the Tooth brewing company. This was because John Tooth, the founder of the business, came from the county of Kent (and indeed the main brewery of the company was known as the Kent Brewery) and the emblem of Kent is a stallion. This came from the Saxons who invaded England after we Romans decided that we had other things to do.

Understandably then the emblem was incorporated into building three's facade when it was erected in 1914.