"A large beast of the purest white, with a thick mane and a strange little beard under the chin, but the most astonishing feature is the long wound horn protruding from its forehead."

The word 'winding' would be more correct.

This is a subtle minor point, but the word "wound" does not scan here (my initial thought was 'injury' not 'winding' and I thought the wrong word had been used entirely).