Samuel Wilberforce was an English bishop who opposed Darwin and evolution. Wikepedia tells us:

He was killed on 19 July 1873, by the shock of a fall from his horse near Dorking, Surrey. Huxley reportedly commented that Wilberforce's brains had at last come into contact with reality, and the result had been fatal.
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I believe there could be a linking between 'Wilberforce' and 'horse' which lasted long enough in folk memory to lead to this silly song.